[Koha] Strange behavior: NO_LIBRARY_SET

Hi guys! This list has solved all my problems until now. So let's go again :) It's not a terrible error but even if I choose a library in logon page, I get a 'NO_LIBRARY_SET' message on the right top corner of my screen? When I try to do something like a search, Koha asks me to set the library. Does someone know what is this? Thanks -- Sistema de Antivirus do CEFET-MG: Esta mensagem foi verificada pelo sistema de antivirus e acredita-se estar livre de perigo.