As I'm getting 504 error trying to open on BM 3.8.5 server, I've compared its packet trace with the other BM 3.8.5 server that can open that page.

On the problematic server there is nothing in the trace after the following message:

Host: . .

On the opposite, on the other server the same message is:

Host: . . Connection: Keep-Alive

and the page loading starts after that.

Does that trace difference direct to some differences in parameters configuration on those two BM servers?