Hi @marcel.f as your bug report mentions TWRP, I thought I would link this in case you had not read it. Gigaset GS290, how to root and install ZIP files? - #32 by aibd
(Note, that walkthrough is for a rescue. When just experimenting with TWRP you would not be doing such a thorough wipe of data and internal storage without a good reason. TWRP also allows a backup, so this is good to checkout first, remembering you have an “experimental” TWRP)