Some info about failed experiments to fix the problem for science:
-wandering out to different zones, then reentering and trying to cross to the zone with the transition error again: did not work
-turning off "autosave on transitions" in option: did not work
-restarting the game: did not work
-restarting the computer: did not work
-uninstalling then reinstalling underrail to another drive and folder: it worked!
I uninstalled Underrail via steam, then reinstalled it. The first 2 times I tried to reinstall, steam would not complete the installation, there was some kind of error in the finalizing stage.
I then tried to install the game to C: instead of D:. Miraculously, not only was the game installed correctly, but I reloaded my previous save that contatined the transition error, and it worked. I hope this can help others who encounter the same issue.
If it is of interest, I can also share that I have over 1000 hours in Underrail, and have never had this issue before. There are no active virus scanning on this computer to interfere with autosave either.
However, The zone in question that I could not transition to, "ForlornStations4", is connected to by the lower level, one of the new, random dungeons: "Zone: dun_lowLevelBunker1". Perhaps this is connected to the error, that there was a hiccup when generating the random dungeon or something - it is the only thing that is different from the other ~50 times I have entered this area for the first time?