The version of pioneer2.irp that is in the zip distribution matches the one reported here:

[Link: remotecentral.com]

I believe the content of the FORM section is wrong:

Form=;*,D:8,~D:8,P:8,~P:8,_,*,S:8,~S:8,F:8,~F:8,_

Compare it with the one used here:

[Link: remotecentral.com]

Form=*,D:8,~D:8,P:8,~P:8,_;*,S:8,~S:8,F:8,~F:8,_

I think there is a good chance that this is the cause of the trouble, but I don't have time to try it right now.