Hi Andreas,
When a PTF for an Adagio product is installed, it looks at the file date and version of the currently installed program. If it finds something that it isn't expecting, such as an incorrect file date, then the message you received will be displayed.
To start with, what version of Receivables do you have installed? You can check this from the Help menu and About. To install this PTF, you need to have a version 6.8A of Adagio Receivables; it will not work with version 6.5A.
My guess from your records is that you do have 6.8A installed and that should be OK, but please verify this.
If the date of the main program file (crwin.exe) has changed from what it was when installed, then this will also prevent installation. What is the date of the file crwin.exe that you are using?
Are you aware if any special program files were sent to you to fix a specific problem? If so, then you may have a backup of the original crwin.exe that you could restore.
When installing this newest PTF, did you also just install any other upgrades for Receivables? It may be that you have to run the program once before the PTF install will recognize the version. This was a problem a few years ago that should be fixed, but give this a try just in case.
Hopefully one of these things should help; please post some more info regarding versions, file dates, etc should the install continue to fail.