I've got an update, it seems that this bug happens for either special branch of the Romulans, locking me out of both of their campaign routes. There is no way around this, as forefeiting the mission results in explusion from the romulan special ops branch you chose.
The issue is not caused by approaching the base, as it seems to simply crash after an almost random amount of time in the mission. I believe it to occur in real time rather than game time (turning the game speed up to 11 seems to buy some time).
I still haven't been able to work out a fix.
Have you contacted GOG support yet? I cannot help you with this, but maybe this is a known problem.