Nextcloud
Nextcloud is a self-hosted file-sync application written in PHP. We've used it pretty heavily for many years now (and ownCloud, its predecessor, before that). Despite being a pretty nifty package in a lot of ways (especially as a concept), it increasingly has lots of little problems that are making it more and more difficult to use and maintain.
Issues
In no particular order:
Upgrade Button Unresponsive
When there is an upgrade available, the "Open updater" button on this page --
-- will often be unresponsive for a long time, even after all the security checks (above it on the page) have finished. Usually if I go off and do something else for about 10 minutes, it will become live and I can then start the upgrade. This has been a problem through at least a few major versions now.