SignTool unexpected internal error 0x80072eff

In our buildscripts we have to sign about 500 assemblies (*.dll, .Net if it is interesting?) with the signtool. Often it runs in this error:

1>EXEC : error information: "Error: SignerSign() failed." 
  (-2147012865/0x80072eff) [F:\Builds\7\profahr\Dev - CI\Sources\_BuildScripts\CodeSigning.proj]

1>EXEC : SignTool error : An unexpected internal error has occurred. 
  [F:\Builds\7\profahr\Dev - CI\Sources\_BuildScripts\CodeSigning.proj]

We try signing assembly by assembly AND all assemblies with placeholder *.dll. Same Problem. Approximately every fifth build process runs into an error after signing 100 to 200 assemblies. Where can I found detailed Information about this internal error?

Thanks for any useful Information Matthias