I'm also having this problem. Only with this tracker. It started about 1 week ago. Using qBitorrent 4.6.4.
My error is "skipping tracker announce (unreachable)".
Happening with 200-544 different torrents throughout the day. Always different number of torrents. Always same error. Always and only this tracker.
Restarted my client 2 hours ago. Right now, 502/545 have connection errors. Keeps changing.
It seems a little better after unchecking IPv6 in Windows Network settings. Now, instead of "unreachable", I get connection timeouts.
Regardless, there's always some amount of errors when I check my client during the day. The problem is still not solved for me either.
@tnar I haven't had any connection errors in the last 12 hours. I've been checking since 6:00 am (CST) this morning. Hope it stays this way.