Thank you for your response!
I typed in your suggested commands. However, I still can’t log in. I get the following error message in the nextcloud.log (I exchanged the IP and the username with ***):
{“reqId”:“fJpuwRggvwYbDTBZzw8S”,“level”:3,“time”:“2021-07-22T22:11:24+02:00”,“remoteAddr”:“217..185.”,“user”:“.”,“app”:“index”,“method”:“GET”,“url”:"/index.php/apps/dashboard/",“message”:{“Exception”:“OCP\Files\GenericFileException”,“Message”:“file_put_contents failed”,“Code”:0,“Trace”:[{“file”:"/var/www/nextcloud/lib/private/Files/SimpleFS/SimpleFile.php",“line”:109,“function”:“putContent”,“class”:“OC\Files\Node\File”,“type”:“→”},{“file”:"/var/www/nextcloud/lib/private/Template/IconsCacher.php",“line”:146,“function”:“putContent”,“class”:“OC\Files\SimpleFS\SimpleFile”,“type”:“→”},{“file”:"/var/www/nextcloud/lib/private/Template/SCSSCacher.php",“line”:351,“function”:“setIconsCss”,“class”:“OC\Template\IconsCacher”,“type”:“→”},{“file”:"/var/www/nextcloud/lib/private/Template/SCSSCacher.php",“line”:189,“function”:“cache”,“class”:“OC\Template\SCSSCacher”,“type”:“→”},{“file”:"/var/www/nextcloud/lib/private/Template/CSSResourceLocator.php",“line”:111,“function”:“process”,“class”:“OC\Template\SCSSCacher”,“type”:“→”},{“file”:"/var/www/nextcloud/lib/private/Template/CSSResourceLocator.php",“line”:63,“function”:“cacheAndAppendScssIfExist”,“class”:“OC\Template\CSSResourceLocator”,“type”:“→”},{“file”:"/var/www/nextcloud/lib/private/Template/ResourceLocator.php",“line”:79,“function”:“doFind”,“class”:“OC\Template\CSSResourceLocator”,“type”:“→”},{“file”:"/var/www/nextcloud/lib/private/TemplateLayout.php",“line”:330,“function”:“find”,“class”:“OC\Template\ResourceLocator”,“type”:“→”},{“file”:"/var/www/nextcloud/lib/private/TemplateLayout.php",“line”:238,“function”:“findStylesheetFiles”,“class”:“OC\TemplateLayout”,“type”:"::"},{“file”:"/var/www/nextcloud/lib/private/legacy/OC_Template.php",“line”:183,“function”:"__construct",“class”:“OC\TemplateLayout”,“type”:“→”},{“file”:"/var/www/nextcloud/lib/public/AppFramework/Http/TemplateResponse.php",“line”:210,“function”:“fetchPage”,“class”:“OC_Template”,“type”:“→”},{“file”:"/var/www/nextcloud/lib/private/AppFramework/Http/Dispatcher.php",“line”:172,“function”:“render”,“class”:“OCP\AppFramework\Http\TemplateResponse”,“type”:“→”},{“file”:"/var/www/nextcloud/lib/private/AppFramework/App.php",“line”:157,“function”:“dispatch”,“class”:“OC\AppFramework\Http\Dispatcher”,“type”:“→”},{“file”:"/var/www/nextcloud/lib/private/Route/Router.php",“line”:302,“function”:“main”,“class”:“OC\AppFramework\App”,“type”:"::"},
{“file”:"/var/www/nextcloud/lib/base.php",“line”:993,“function”:“match”,“class”:“OC\Route\Router”,“type”:"->"},{“file”:"/var/www/nextcloud/index.php",“line”:37,“function”:“handleRequest”,“class”:“OC”,“type”:"::"}],“File”:"/var/www/nextcloud/lib/private/Files/Node/File.php",“Line”:73,“CustomMessage”:"–"},“userAgent”:“Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:90.0) Gecko/20100101 Firefox/90.0”,“version”:“21.0.3.1”}
{“reqId”:“fJpuwRggvwYbDTBZzw8S”,“level”:3,“time”:“2021-07-22T22:11:24+02:00”,“remoteAddr”:“217..185.”,“user”:“.”,“app”:“PHP”,“method”:“GET”,“url”:"/index.php/apps/dashboard/",“message”:{“Exception”:“Error”,“Message”:“file_put_contents(/media/usb/nextcloud/data/appdata_ocmdkw09qk9w/css/icons/icons-vars.css): failed to open stream: Input/output error at /var/www/nextcloud/lib/private/Files/Storage/Local.php#282”,“Code”:0,“Trace”:[{“function”:“onError”,“class”:“OC\Log\ErrorHandler”,“type”:"::"},{“file”:"/var/www/nextcloud/lib/private/Files/Storage/Local.php",“line”:282,“function”:“file_put_contents”},{“file”:"/var/www/nextcloud/lib/private/Files/Storage/Wrapper/Wrapper.php",“line”:259,“function”:“file_put_contents”,“class”:“OC\Files\Storage\Local”,“type”:"->"},{“file”:"/var/www/nextcloud/lib/private/Files/View.php",“line”:1170,“function”:“file_put_contents”,“class”:“OC\Files\Storage\Wrapper\Wrapper”,“type”:"->"},{“file”:"/var/www/nextcloud/lib/private/Files/View.php",“line”:707,“function”:“basicOperation”,“class”:“OC\Files\View”,“type”:"->"},{“file”:"/var/www/nextcloud/lib/private/Files/Node/File.php",“line”:72,“function”:“file_put_contents”,“class”:“OC\Files\View”,“type”:"->"},{“file”:"/var/www/nextcloud/lib/private/Files/SimpleFS/SimpleFile.php",“line”:109,“function”:“putContent”,“class”:“OC\Files\Node\File”,“type”:"->"},{“file”:"/var/www/nextcloud/lib/private/Template/IconsCacher.php",“line”:146,“function”:“putContent”,“class”:“OC\Files\SimpleFS\SimpleFile”,“type”:"->"},{“file”:"/var/www/nextcloud/lib/private/Template/SCSSCacher.php",“line”:351,“function”:“setIconsCss”,“class”:“OC\Template\IconsCacher”,“type”:"->"},{“file”:"/var/www/nextcloud/lib/private/Template/SCSSCacher.php",“line”:189,“function”:“cache”,“class”:“OC\Template\SCSSCacher”,“type”:"->"},{“file”:"/var/www/nextcloud/lib/private/Template/CSSResourceLocator.php",“line”:111,“function”:“process”,“class”:“OC\Template\SCSSCacher”,“type”:"->"},{“file”:"/var/www/nextcloud/lib/private/Template/CSSResourceLocator.php",“line”:63,“function”:“cacheAndAppendScssIfExist”,“class”:“OC\Template\CSSResourceLocator”,“type”:"->"},{“file”:"/var/www/nextcloud/lib/private/Template/ResourceLocator.php",“line”:79,“function”:“doFind”,“class”:“OC\Template\CSSResourceLocator”,“type”:"->"},{“file”:"/var/www/nextcloud/lib/private/TemplateLayout.php",“line”:330,“function”:“find”,“class”:“OC\Template\ResourceLocator”,“type”:"->"},{“file”:"/var/www/nextcloud/lib/private/TemplateLayout.php",“line”:238,“function”:“findStylesheetFiles”,“class”:“OC\TemplateLayout”,“type”:"::"},{“file”:"/var/www/nextcloud/lib/private/legacy/OC_Template.php",“line”:183,“function”:"__construct",“class”:“OC\TemplateLayout”,“type”:"->"},{“file”:"/var/www/nextcloud/lib/public/AppFramework/Http/TemplateResponse.php",“line”:210,“function”:“fetchPage”,“class”:“OC_Template”,“type”:"->"},{“file”:"/var/www/nextcloud/lib/private/AppFramework/Http/Dispatcher.php",“line”:172,“function”:“render”,“class”:“OCP\AppFramework\Http\TemplateResponse”,“type”:"->"},{“file”:"/var/www/nextcloud/lib/private/AppFramework/App.php",“line”:157,“function”:“dispatch”,“class”:“OC\AppFramework\Http\Dispatcher”,“type”:"->"},{“file”:"/var/www/nextcloud/lib/private/Route/Router.php",“line”:302,“function”:“main”,“class”:“OC\AppFramework\App”,“type”:"::"},{“file”:"/var/www/nextcloud/lib/base.php",“line”:993,“function”:“match”,“class”:“OC\Route\Router”,“type”:"->"},{“file”:"/var/www/nextcloud/index.php",“line”:37,“function”:“handleRequest”,“class”:“OC”,“type”:"::"}],“File”:"/var/www/nextcloud/lib/private/Log/ErrorHandler.php",“Line”:92,“CustomMessage”:"–"},“userAgent”:“Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:90.0) Gecko/20100101 Firefox/90.0”,“version”:“21.0.3.1”}