From your messages, you sound quite knowledgeable about your system, so my comments may be also come across as offensive.
My first thoughts are about the mechanical connections of the batteries. From your description, it sounds like you are running two strings of batteries creating a 12v system. Could it be possible that one of the six Surrette have a loose connection?
I’m trying to picture what will happen if the Outback controller saw unequal strings of batteries (OR if it only saw two well-charged 4v batteries). It might “Think” you have one very weak 12v battery, and work very hard to bring voltage back up. The boil-over could be a sign of possible over-charge, or it could be automatic equalization doin’ its thing.
If all the connections look good, I would suggest breaking the batteries apart and only use 3 batteries to create the 12v. Let that run a few days, then try the second set of batteries. You might find one bad apple in the mix.
Again, take my suggestion with a grain of salt…. I’m still running a small system and using Deep-Cycle Marine batteries. (my own way to learn-the-hard-way about battery care)
Best of luck
