It took me two weeks of troubleshooting, cross-compiling binaries and reverse engineering the source code, until I figured out that I wasn't doing anything wrong - Synology had actually changed their version of sshd to add all sorts of proprietary crap [2].
Another one was the "Mouse battery low" alerts on Gnome under Ubuntu [3]. The root cause was in upowerd, but Gnome developers could have applied more sane defaults (e.g., no need to show the same alert several times per day, and even worse considering they don't go away unless you click to dismiss). Certainly not 100 hours, but much more time than I'd like.
[1] https://serverfault.com/questions/458553/cant-log-in-via-ssh...
[2] https://wrgms.com/reverse-engineering-synology-openssh/
[3] https://wrgms.com/disable-mouse-battery-low-spam-notificatio...