Hello @moreforth, welcome to the community of Nextcloud.
You started a topic in support category. Unfortunately you ignored the template and a lot of information to help you is missing. Please add all necessary information like Nextcloud version, webserver type and version, os version, related log file content. Use the support template.
Use search many topics have been discussed already! As the system seems to works and just the display is wrong this might be related to some dark theme issue like discussed here: Can't see PDF files - #3 by wwe
Regards,
wwe