MyPassport-SSD to thunderbolt / USB3.1 interface is not recognized

I recently bought the SSD “My Passport” (256 GB) and wanted to connect it to the thunderbolt / USB 3.1 interface of my laptop XMG-U706 (by Schenker).
Unfortunately, it was not recognized on the (combined) thunderbolt / USB 3.1 USB port, while the SSD is running on the other (slower) USB 3.0 interfaces.
In between, the SSD was strangely recognized for a day at this interface - but then no more.
I urge you for advice - thank you!
(I’m German - excuse my english)