I had a similar issue 2 days ago, where users were not able to login anymore, cause of out-of-date virus definition files. I decreased the interval timer for the download from 120 to 60 minutes and tested in the evening again. Everything worked then. But I didnt get an error that the IVE was unable to download the definition files.
One question regarding this: Why is the max. age of a definition file only 10 "updates"? I remember in older versions it was possible to define the number of days the definition files are accepted.
Changing the update timer from 60 minutes to one minute appears to correct the issue and users can connect. We change the timer back and the issue returns in about 10 days.