"Buffer Underflow" crash bug, persistent error and cannot log in to character

I was playing in Cruel Act 3 with my Gemling Legionnaire (minion build) in Jungle Ruins when I suddenly had a "Buffer Underflow" error message and POE2 hanged, then closed when I clicked "OK".

Can re-start the POE2 client (am playing on the stand-alone client), and log on with other characters.

But once I try to log back in to my Gemling Legionnaire, the loading screen for Jungle Ruins appears, but then after that I get the same Exception message and a "Buffer Underflow" message and the game client freezes immediately, and then closes itself.

Completely unable to log back in to my Gemling Legionnaire (my current main character).

Managed to log in 30ish minutes later to a previous checkpoint!