Emily
I am still having this problem. I connect to VPN and then Tor with Tor Browser and get errors showing proxy error after a while.
It's happening enough I think someone is trying to track me through my phone and I'm ready to throw it away. Prior to Graphene OS I did not have a cell phone and I don't trust cells at all and don't even trust Graphene OS on a Pixel that much because I am afraid of Pixel hardware.
I do not have another phone or another person to use authenticator. I have cleared cache and all storage on Tor and the VPN and am not on an end of life model, everything is up to date. I am getting these proxy errors with no site loading often even if changing curcuits. This never used to happen, I have a strong connection, there is no explanation for this except being hacked by an advanced adversary. The sites getting this error accept all Tor connections and are not illegal or dark web sites but I may have a higher than average threat model.
I'm ready to throw my phone away unless someone helps me understand what's happening.