After updating Spark source with Smack 4.3 (thanks, flow) it will start and login correctly, but it opens blank chat windows in roster and it doesn't show your messages as sent in MUC (though it actually sends them). If one deletes plugins directory from Spark profile, it works fine. So this is old plugins (some of them even won't be updated as there are no maven artifacts for them) causing troubles. But users updating from older versions of Spark will have them in their profiles. So marking this as a blocker. Either something has to be done for Spark to maybe ignore such incompatible plugins. Or maybe we can somehow remove them from profiles. Second is harder, so first solution is probably better.
After more testing we have found out that it's the OTR plugin causing this.