I had to focus on other projects and couldn't follow through the diagnosis of this unit, now I have some time to get to the bottom of it.
I did the first step, reverted back to RUT850_R_00.01.03_WEBUI without keeping settings and the Mobile page loaded with no errors.
Then I upgraded again to RUT850_R_00.01.04_WEBUI and the error reappeared. No changes to the configuration were made.
I have been in touch with the RMA people of my Teltonika distributor, and told me they had several instances of this error which was solved by this procedure, unfortunately it fails on my unit.
Additionally the unit will lose connectivity after 8-24 hours since boot and stays disconnected. I had to set up an automatic modem reboot after failed pings, as described in this other thread, but this seems a crutch for a failing hardware/firmware.
We have deployed several RUT240/RUT950 and didn't have to set up such rule.
I will send you the troubleshooting file via PM