I recently had to upgrade two log insight instances for a customer from 4.x to 8.x. This was a manual process in two phases (you have to upgrade to 4.8 first). After the upgrade Log Insight was running, but we saw intermittent collection/task error messages on the vCenters. (Every instance connects to 4 vCenters).
I found 2 KB articles that solved our problem. If you upgrade it is probably worth taking a look at them. These issues were not documented in the release notes as far as I know.
The first is around WCP collection for Kubernetes. (KB 78107). This is apparently enabled by default after a 8.x upgrade ! Follow the KB to disable it in the advanced config.
The second one occurs because Log Insight can no longer update time stamps on the files it keeps for every vCenter. Follow the KB 70633 and you wil see if files have an old time stamp.