When things aren’t ticking perfectly for Synology their software can be kind of weird. Sometimes after power failures, some disks get corrupted and… you simply can’t log in to the Synology UI during this time (unless you “synobootseq --set-boot-done”, why, of course) for an unspecified number of hours.

Their custom software has its quirks (eg scp doesn’t work unless you apply the -O flag, for “security” reasons), and the quirks change sometimes after updates.