I had a similar issue with 3 RUT955 devices. It did not check error logs, but noticed that all certificate and key files had size 0kb after RUT9_R_00.07.02.6. File names were still correctly displayed. After uploading ca, user crt+key and hmac key again, openvpn connections worked again.
It happened on 3 out of approx 20 devices during updates.
The 1 of the 3 devices had previous version 6.9.2 and the 2 others had 7.1.4.
Updates of the other, almost identical 20 devices from same firmware versions went without any issues.
I suspect that the firmware update sometimes overwrites the memory space of the vpn files, or somehow messes up the file system.
I cannot provide you the toubleshoot files of these 3. If it happens again, I will try to remember.