The correct spelling for Ernst Thälmann is only with one L. Also the street signs in Varnsdorf are correcty written "Ernsta Thälmanna".

Now, I could of course just change it here in OSM, but the source of the wrong spelling seems to be ČÚZK, because they also spell it with two L. So I think it is best to leave it to the locals to decide what to do with this. I assume the wrong spelling is automatically imported from ČÚZK somehow, so somebody should probably contact ČÚZK about this to have it fixed?