Hey,
from what i have read in the past ownCloud uses basic authentication on its own. Maybe the one of your web server is conflicting with the one of ownCloud in newer versions?
Update which could confirm the above:
I did the following search Search results for 'basic auth' - ownCloud Central and found the following commend in an existing thread:
Hi,
it is known that you can't use an additional Basic Auth in front of ownCloud for the simple reason that oC is using an own Basic Auth in various functions like those shared links etc.
You can't have two basic auths (one by apache and one by ownCloud's WebDAV). If it has worked, it worked by luck, as the behavior of a daisy chained basic auth is undefined.
-> https://github.com/owncloud/client/issues/1618#issuecomment-38775347
Unfortunately iām not sure if this also applies for the login page itself
