I have recently observed a repeated situation where many (or all) threads have been stuck waiting for credits in FlowControl protocol.
The credit request was not handled on the other node as this is non-oob message and some (actually many of them - cause unknown) messages before the request have been lost - therefore the request was waiting for them to be re-sent.
However, these have not been re-sent properly as the retransmission request was not received - all OOB threads were stuck in the FlowControl protocol as these handled some other request and tried to send a response - but the response could not be sent until FlowControl gets the credits.
The probability of such situation could be lowered by tagging the credit request to be OOB - then it would be handled immediately. If the credit replenish message would then be processed in regular OOB pool, this could get already depleted by many requests, but setting up the internal thread pool would solve the problem.
Other consideration would be to allow releasing thread from FlowControl (let it send the message even without credits) if it waits there for too long.
It appears that setting MFC and UFC max_credits to 10M or removing these protocols at all is a workaround for this issue.