[Divingduck]

@chaley,
Thanks for your answer and alternative solution. I will check out your PI's. The performance issue was known and that's why I use the functionality/column only if I need it.

If you want, I can send you the DB with 7 books for testing. Maybe this helps you and Kovid locating the problem. When I switch between libraries, sometimes the error changed to "These function can only use in the GUI" and in this case I became a template error on other columns build from other columns (e.g. {author_sort}). When I open the VL-column in the template editor I became the old error message back. Curious, it looks like there is running something out of boundary.