dotPeek cannot find an assembly in the GAC and is unable to generate pdb

I configured VS 2017 to use dotPeek as a symbol server, but dotPeek is unable to generate some pdbs.

The status message is: "Unable to generate pdb because the assembly with pdb name Xxx%20Yyy%20Zzz and signature ... is not found."

I'm pretty sure the assembly is in the GAC and its signature is correct, in fact it is no different from many other assemblies that dotPeek handles correctly.

Maybe dotPeek gets confused by the spaces in the name that are translated into "%20"?
Are there other possible explanations?