Hi Steven, 

Make sure that Assemblybindings in site web.config corresponds to what you have for Mirroring_Web.config.

Thanks Shahid.

Actually I had copied the entire <assemblyBinding> section from site web.config to mirroring web.config. Though this also includes <probing privatePath="modulesbin" />.

What is strange is that mirroring will run fine for a few days and then the error will show up from out of nowehere and then be present preventing mirroring from running until I've restarted both site and mirroring instance.

I am running EpiServer.mirroring.dll version 7.5.394.2 in both site and mirroring-instance bin folders.

You dont' need to copy all the assembly bindings from the site's config. Just update the binding redirects in mirroring web.config which were there before you upgraded the site. Also make sure that there is no mismatch between the assemblies in site' bin folder and mirroring bin folder on both source and Target sites/mirroring services. 

Do the following steps: 

1- Kill mirroring w3wp process (both source and target mirroring) from task manager.

2-Run all four applications in the web browser like : 

   www.sourcsite.com
   www.sourcesite/MirroringSourceServer.svc
   www.targetsite.com
   www.targetsite/MirroringTransferServer.svc

3- Enable EPiServer log on "DEBUG" level on both source and target mirroring services (copy the episerverlog.config from site root and paste it under MirroringService root folders)

4- On source site , open the mirroring channel in admin mode and press "Check System". You should be able to see details in either source mirroring log file or target mirroring log file. 

There hasn't been any changes made to MirroringService itself since the EPiServer 7.5 release so the assembly version for EPiServer.Mirroring.dll remains  7.5.394.2

Thank you! I'll give that a bash, its running ok now so I'll need to wait for the error to appear again.