Same issue... The bug hunting session continues...
Here is a new patched binary which adds safe guards and debug messages to the mesh repository code (I think this might be a problem with a stale mesh request reply getting triggered while the request class itself has been already destroyed, or a bad (empty) LOD data received, both being capable of causing a crash during the mesh optimization).
Please, download it and overwrite the old patched binary with it, then start the viewer, login, and in "Advanced" -> "Consoles" select "Debug tags" and in the floater which appears scroll down to check the "Mesh" item. Then try and make the viewer crash... If in the log you see warnings about "Stale response received for removed instance. Ignoring." or "No data received for mesh Id: .../... - LOD: n", then it means we caught the issue and you should not crash any more... If you still crash, well... you now the drill now: post the log and crash dump, as usual.