https://bugs.winehq.org/show_bug.cgi?id=39602
--- Comment #9 from winetest@luukku.com --- (In reply to mazhe from comment #8)
I cannot say for sure, if I disable only the assert in jsval_copy(), there's no crash, but the downloading interface do not load completely (no news, no download)... I don't think this was the same issue though.
Bug 38980 has workarounds for the launcher crashes. If they work for you then this bug is a dupe.