It's exactly what was discussed in #2571, which was fixed in 5.1.7 by 84ac0c4.

After upgrading to 5.1.9, the same problem occurs again. It could possibly be broken in 5.1.8, but there is no way to know since proxy mode had that critical bug in 5.1.8.

Update: I tried VPN mode in 5.1.8, and tested the connectivity, it was most likely broken already in 5.1.8. I have tried every version afterwards, the result is the same.