compatibility problem - CS5.5 plugin doesn't work with CS5.5

Hi,

I have a problem with our CS5.5 plugin, it works without problems one my machine but the customer gets the errormessage "Adobe InDesign does not recognize JOURNALDESIGNERPLUGIN.PLN as valid plug-in. Please reinstall...."

I know this message from accidentaly putting a CS5 plugin in the CS5.5 folder - but this is a CS5.5 plugin. I tried exactly the same plugin file the customer used in my InDesign and it works here. We both use version 7.5.3 on Windows.

And ideas where the problem is?

Thanks

Klaus