SDLXLIFF file created from TXLF file – File type not supported

Hello all – and SDL Studio experts in particular!

A client regularly sends me txlf files. I have added this file type under Options/File Types/XLIFF for the format to be recognized. However, during my current project I re-installed Studio 2021 in a (futile) attempt to resolve another Studio 2021 problem (Object reference not set to an instance of an object, see my previous post). After doing so, I added the .txlf file type again. However, Studio will no longer recognize the sdlxliff file it saves for this project. I have tried opening from within Studio, via the project file and by clicking on the saved sdlxliff file.
Same error every time: This file type is not supported.


Current version of SDL Studio: SDL Trados Studio 2021 SR1 - 16.1.2.4090
Current workaround: Start from scratch with original txlf file every morning, and rely on TM to fill in work already done.
Workaround from now on: Do not close file until translation is finished.
Impact on productivity: severe.

Any tips and  advice welcome!