Dear fordiegolg,
welcome to the club, you're absolutely right that there seems to be a severe bug in this version (8.x and AEA).
We've already had the same problem like you with the mirror which should be fixed now i believe.
And we still have the same problem that no system can be patched anymore due to Microsoft's Meltdown requirements and
its registry key AND Avast.
Beside WSUS Online we're using the offline wsusscn2.cab in 3rd party applications or direct api calls with WUA, the online WSUS version is working for some cases but the offline one for absolutely NO system.
And you're right it's definitely the Avast scanner, registry keys are ok, you can disable the services and disable feature of the realtime scanner, nothing is working. Only the old non-flagged patches were recognized and installed.
Because Avast's business support is sadly blazing with abstraction since the merge with AVG we've opened a case at Microsoft Premier to probably narrow it down more in detail. Nevertheless you're right, if uninstalling Avast all is working without any problems - also for testing with other av vendors. Btw. we're still using it on about 1500 servers. Still waiting on the final results of the L2/L3 engineers of Microsoft support. Will get them probably today or at monday... (btw. all server operating systems are affected, not only the client versions..)