FYI, I think we completely fixed it now in v5.7.3.
v5.7.3 is now live on the web site and nuget.
It turned out the error was due to a stupid bug:
The log file AssemblyResolver.log was locked when you first started your web app as expected but even if you restarted the web app it could stay locked.
Then during the next web app initialization, AssemblyResolver would fail initialization because it could not overwrite the log file AssemblyResolver.log.
It was hard to replicate because the file did not stay locked after every restart/recycle.