TFS: “Could not load type ' ' from assembl

Question

In my solution, I run my unit tests and they all pass. When I then commit via TFS then sometimes the following shows in the TFS build step regarding one of the projects:

Could not load type 'Castle.Proxies.HttpContextProxy' from assembly
'DynamicProxyGenAssembly2, Version=0.0.0.0, Culture=neutral,
PublicKeyToken=xxx'

Why is the error only showing sometimes?

I've checked all the Assembly Names under 'Application' tab of properties in VS as suggested in "Could not load type from assembly error" - also tried to clear my bin-folder, but nothing happens - still errors show occassionaly in the tfs-build steps for one of the projects unit test (which again never shows in VS17).