Rainer,
You should not have to have an external power supply for your LimeSDR because I’ve always used my V1.2 and V1.4 boards with just USB 3.0 power. But there is hope - - PLEASE refer to this post and read ALL the threads from it. This LimeSDR user @cswiger had a VERY similar issue to what you’re seeing and finally got it fixed. Here’s the link to that post:
Again, please read ALL the threads in that post and see if you can recover. As well, stay in touch - we’ll get you on the road to recovery…
73 de Marty, KN0CK