Yep, mod_security was the problem. Disabled rule 214940 so the admin-redirect was solved (seems to be caused by wp-emoji-release.min.js)
Disabling rule 214560 solved 403-errors for our clients.
In our case there was no need to disable 214620.
Same problem here, multiple websites have a redirect loop for /wp-admin/. Some websites don’t. Opening /wp-admin/index.php does work without trouble.
Siteurl is lowercase and switching to default theme and/or disabling plugins does not solve the problem.
.htaccess-file has only default lines in it.
Problem occurs on different servers and different hostingproviders. The same servers are running websites which don’t have the problem as wel, so it cannot be a problem with the server configuration.
