Bisection: (argb codec needed revision r13763 to build on win32 to test)
Sub-bisecting the large r13756 changeset as there's no obvious problem with it:
Sub-bisecting "platform" module:
Sub-bisecting that, leads to this tiny innocuous change as being the source of the problem: https://github.com/Xpra-org/xpra/commit/fd7d524e1568cd3edd826794584d42700e1374ac"xpra.util" from "xpra.platform.win32" seems to have some bad side-effects, so r14111 delays the import until we actually need it and that fixes the problem. I don't really understand why, but I don't have time to look into it either.
this ticket has been moved to: https://github.com/Xpra-org/xpra/issues/1337