That would appear to be this issue:
For More Information see: External Data Source Text Field Truncated to 8192 chars when viewed in layout field.
This is one of many acknowledged bugs that can be found in the Known Bug List here in the Report an Issue section of the forum.
It can also be downloaded as a database file from: http://www.4shared.com/file/8orL8apk/FMP_Bugs.html
Thanks for your answer. Allthough it is a pity, I now can stop searching for a solution for this problem, hoping that Filemaker takes care of it soon. Therefore I also submitted a feature request.