Just from the error message that you are getting, it doesn't look like it is related to the removal of the log4net dependency.
Trying to figure out what is happening, the dependencies listed for EPiServer.Find.UI.FindInitializationModule are:
IndexingModule being part of EPiServer.Find.Cms (EPiServer.Find.Cms.Module.IndexingModule) and versioned together with the rest of Find and ShellInitialization being part of EPiServer.Shell.
Could it be that the assembly binding redirects in web.config are incomplete or have the incorrect version numbers (either in the ranges or the version to use)? Ie, maybe it can't find ShellInitialization?
These redirects should be set up appropriately during package installation but if that has failed somehow or if web.config has been replaced afterwards it seems like one possibility that could lead to this kind of issue.
Weird, I was about to check the assembly bindings after reapplying the Update 66, but now everything works. So appearently I must've done something wrong last time.
Thank you for your help.
After updating the NuGet packages to the latest Update 66, from installed Update 64, I get the following error when accessing any page:
Cannot resolve dependencies for the following module(s)
I understand the Find-update in v66 changed the dependency to log4net. Is this something related to that, and what can I do about it?