The same problem with me, I played on the 1.6 version, everything worked like a clock, after the launcher stopped finding the server, I tried to demolish everything and put it in a new way, but already on version 1.5, the problem remained. Today I will try to remove the configuration files of the game itself as written above, it may help.
Edit1: I reinstall all again , server 1.5.1 , remove old profile , 0 mods , server works but still launcher cant connect.
It may be some kind of problem with my computer, but I don't even know where to dig
