Teracube 2e - update 2.12 coninuously failes

We have three Teracubes 2e (2.11), one of which we bought in March '22, the others in May '23. The first one updated perfectly fine to 2.12, those bought in May download the update, but after restart the system is still 2.11. The phones seem identical, but something must have changed to prevent updating. Can anybody help?