Sadly the workaround I posted about is starting to be less efficient. Before it goes into the charging/restarting loop I sometimes need to remove the USB-C cable multiple times. I fear that I won’t be able to use the FP5 at all in a couple of days.
I will send it in to FP for repair. Hopefully I get enough charge from the current charge cycle to backup data and restore FPOS.
Thank you for your post @Eric85, that shows me, that it might really be software or at least malfunction in the non repairable hardware.