Hi Michał
I suppose problem may be in content of an index - your map of Poland works well on my device but any map compiled by me (using latest mkgmap release) using data I sent you (bigger Europe slice than boundary of Poland) has broken index. Yesterday I made some tests and neither compilation with latin1 not 1252 nor 1250 works. Index in MapSource works well, maps uploaded using MapSource also work well.
I've found a few differences with the gmapsupp that we produce and the attached patch fixes most of them. I would appreciate if you could test the current patch. Trouble is, that nothing that I've changed really appears to be specific to that part of the world and so I don't why it does not affect everyone. There are a few names that begin with spaces, which are possibly a problem. I would like to get rid of all initial spaces from names and I will attempt that in the next patch. Best wishes ..Steve