Welcome from Germany!
About you thread troubles: after doing all the things already suggested I would get NEW thread. I am using 100% cotton thread exclusively, and discovered a great website last year: www.redrockthreads.com. They have a great selection of all the brand names. If at all possible don't use the cotton-covered polyester thread, or polyester for that matter.
I handquilt, using 100% cotton thread too - I guess I am a traditionalist!
Happy quilting! UM from Bavaria