Adding 'use_ssl' => true to the config did not help. I am still first met with:
Storage unauthorized. Authentication failed, verify the username, password and possibly tenant Storage is temporarily not available
And on subsequent attempts only:
Storage is temporarily not available
There’s not much in terms of errors to go on but I found this in the nextcloud.log:
{"reqId":"FOF4LN0zoeBz5lwFEUcG","level":3,"time":"May 27, 2022 07:40:56","remoteAddr":"xx.xx.xx.xx","user":"--","app":"core","method":"GET","url":"/login","message":"","userAgent":"Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/101.0.4951.64 Safari/537.36","version":"23.0.3.2","exception":{"Exception":"OCP\\Files\\StorageNotAvailableException","Message":"","Code":1,"Trace":[{"file":"/var/www/html/lib/private/Files/Storage/Wrapper/Availability.php","line":242,"function":"checkAvailability","class":"OC\\Files\\Storage\\Wrapper\\Availability","type":"->","args":[]},{"file":"/var/www/html/lib/private/Files/Storage/Wrapper/Wrapper.php","line":227,"function":"file_exists","class":"OC\\Files\\Storage\\Wrapper\\Availability","type":"->","args":["appdata_ocif5mryemvh/js/core/merged-template-prepend.js"]},{"file":"/var/www/html/lib/private/Files/View.php","line":1344,"function":"file_exists","class":"OC\\Files\\Storage\\Wrapper\\Wrapper","type":"->","args":["appdata_ocif5mryemvh/js/core/merged-template-prepend.js"]},{"file":"/var/www/html/lib/private/Files/View.php","line":1393,"function":"getCacheEntry","class":"OC\\Files\\View","type":"->","args":[{"__class__":"OCA\\Files_Trashbin\\Storage","cache":{"__class__":"OC\\Files\\Cache\\Cache"},"scanner":null,"watcher":null,"propagator":null,"updater":null},"appdata_ocif5mryemvh/js/core/merged-template-prepend.js","/appdata_ocif5mryemvh/js/core/merged-template-prepend.js"]},{"file":"/var/www/html/lib/private/Files/Node/Root.php","line":200,"function":"getFileInfo","class":"OC\\Files\\View","type":"->","args":["/appdata_ocif5mryemvh/js/core/merged-template-prepend.js"]},{"file":"/var/www/html/lib/private/Files/Node/Folder.php","line":146,"function":"get","class":"OC\\Files\\Node\\Root","type":"->","args":["/appdata_ocif5mryemvh/js/core/merged-template-prepend.js"]},{"file":"/var/www/html/lib/private/Files/Node/Folder.php","line":155,"function":"get","class":"OC\\Files\\Node\\Folder","type":"->","args":["merged-template-prepend.js"]},{"file":"/var/www/html/lib/private/Files/SimpleFS/SimpleFolder.php","line":71,"function":"nodeExists","class":"OC\\Files\\Node\\Folder","type":"->","args":["merged-template-prepend.js"]},{"file":"/var/www/html/lib/private/Template/JSCombiner.php","line":116,"function":"fileExists","class":"OC\\Files\\SimpleFS\\SimpleFolder","type":"->","args":["merged-template-prepend.js"]},{"file":"/var/www/html/lib/private/Template/JSCombiner.php","line":102,"function":"isCached","class":"OC\\Template\\JSCombiner","type":"->","args":["merged-template-prepend.js",{"__class__":"OC\\Files\\SimpleFS\\SimpleFolder"}]},{"file":"/var/www/html/lib/private/Template/JSResourceLocator.php","line":115,"function":"process","class":"OC\\Template\\JSCombiner","type":"->","args":["/var/www/html","core/js/merged-template-prepend.json","core"]},{"file":"/var/www/html/lib/private/Template/JSResourceLocator.php","line":71,"function":"cacheAndAppendCombineJsonIfExist","class":"OC\\Template\\JSResourceLocator","type":"->","args":["/var/www/html","core/js/merged-template-prepend.json"]},{"file":"/var/www/html/lib/private/Template/ResourceLocator.php","line":78,"function":"doFind","class":"OC\\Template\\JSResourceLocator","type":"->","args":["js/merged-template-prepend"]},{"file":"/var/www/html/lib/private/TemplateLayout.php","line":388,"function":"find","class":"OC\\Template\\ResourceLocator","type":"->","args":[["core/js/dist/main","js/dist/files_fileinfo","js/dist/files_client","js/merged-template-prepend","core/l10n/en","And 13 more entries, set log level to debug to see all entries"]]},{"file":"/var/www/html/lib/private/TemplateLayout.php","line":216,"function":"findJavascriptFiles","class":"OC\\TemplateLayout","type":"::","args":[["core/js/dist/main","js/dist/files_fileinfo","js/dist/files_client","js/merged-template-prepend","core/l10n/en","And 13 more entries, set log level to debug to see all entries"]]},{"file":"/var/www/html/lib/private/legacy/OC_Template.php","line":182,"function":"__construct","class":"OC\\TemplateLayout","type":"->","args":["error",""]},{"file":"/var/www/html/lib/private/Template/Base.php","line":132,"function":"fetchPage","class":"OC_Template","type":"->","args":[]},{"file":"/var/www/html/lib/private/legacy/OC_Template.php","line":298,"function":"printPage","class":"OC\\Template\\Base","type":"->","args":[]},{"file":"/var/www/html/index.php","line":44,"function":"printErrorPage","class":"OC_Template","type":"::","args":["","Storage is temporarily not available",503]}],"File":"/var/www/html/lib/private/Files/Storage/Wrapper/Availability.php","Line":93,"Hint":"Storage is temporarily not available","CustomMessage":"--"}}