Hi Ralf,
OK, I went back through your description and applied the requisite changes to my demo to reproduce your error. As near as I can tell, this is a function of a bug in SAP's code generator. Here, because the key of the KEY_TAB_FOUND_LOCAL internal table was defined generically, you couldn't have a non-character field as part of the key. The workaround I came up with was to redefine the SRTF2 field as a NUMC(10) field so that the key constraints would line up properly. Once I made this change, everything seemed to work for me. Note that you may have to blow away your classes to make this work - the generator seemed to have a hard time cleaning up its mess.
Hope this helps.
Thanks,
James