Maybe it was clear to the rest of the world but
[mdToken=0x6000020] Method does not exist.
doesn't mean that the method with metadata token 0x6000020 does not exist.
It means that in 0x6000020 there is a reference to a method tha does not exist.
Since in previous runs of PeVerify I was getting the exact offsets of the offending operation,
I completely misunderstood the error messages and was looking for the problem in the wrong place
(that is, the places where 0x6000020 is referenced).
The offending line was inside 0x6000020 and specifically this call to an extension method that
was performed as instance method call
IL_0007: callvirt string [PerPlexEd.Core/*23000005*/]PerPlexEd.StreamBinaryExtensions/*01000012*/::ReadBinaryString(class [mscorlib/*23000001*/]System.IO.Stream/*01000007*/) /* 0A000019 */
They should really publish more documentation about PeVerify.
Thanks for trying to help.