First of all, thank you for your fast replay, unfortunately it has been a stressful week for me so i just got to answer now.
I am happy to say that the issue resolved itself. This post is just supposed to help people that are experiencing the same issue.
The fault was caused by the SD-Card.
I just assumed deleting the file system (before the firmware update) would also reformat the SD card, but it did not.
Since the old Disk file associated with the old firmware was still saved on the SD-card witch cased the cam not being recognized.
To unbrick it I disconnected the SD-card and reuploaded the firmware via the SBL mode. After updating the firmware i reformatted the SD-cards, after reinserting the card into the cam it could be recognized again.
This fix appalled to both our cams.