I am on 25.03 but had this error for the last 5-6 updates and decided today is the day I am going to tackle it!
I have an awful lot of files reported as being problematic but searching and searching I cannot find a fix close to my problem? Can anyone please point me in the right direction?
Go to your apps and check Calender version and disable this app, than find corresponding version in a https://apps.nextcloud.com/apps/calendar/releases, download archive, extract it and replace all files in a /path/to/yourNC/apps/calender folder.
Now you can enable app and run integrity check again and see if it solved.
Just to update, I had a corrupt file that could not read/write, I had to format my unraid cache drive and restore a backup. All working now, thank you for trying to help