A small update:
I replaced the script task with a dataflow that uses a database connection and query (select 1 as dummy where 1=0) to produce 0 rows, which are then run through 3 rowcounters for the variables. An awful solution in my opinion...but it fixes the problem on our 64 bit machine for now.
When I have the time I'll dig into tackeling the root problem - 64 bit not being able to compile the code without PreCompile in certain situations. (And determining in which situations the problem occurs at all for that matter)
Thanks again for your input...
Regards,
Dave