"PTY allocation request failed" error when trying to connect using SSH with ControlMaster

I'm trying to log into Ubuntu desktop. I get the following error message:

PTY allocation request failed

What does it mean and how to connect to my desktop?

This is the output with -v:

debug1: Reading configuration data /home/evag/.ssh/config  debug1: /home/evag/.ssh/config line 1: Applying options for *debug1: Reading configuration data /etc/ssh/ssh_config debug1: /etc/ssh/ssh_config line 19: Applying options for *debug1: auto-mux: Trying existing masterdebug1: mux_client_request_session: master session id: 2PTY allocation request failed

Given the output presented above with the verbose option, I think I understand where the problem comes from: I have two computers, A and B. I'm using a controlmaster in A to keep persistent connections to B. A logs into B in a safe way, with command=/bin/false to limit the usage A has on B (it has only forwarding capabilities).

However I would want to connect to ssh from A to B without using this identity (-i), but the problem is that since there is a persistent connection, it uses the existing connection regardless of the option I put in -i, so I end up with not being able to allocate a PTY.

So given that there is an existing controlmaster and protected connection from A to B, how can I get another connection from A to B, but this time with full rights?