Originally, Spaw used language definitions located in the /spaw/lib/lang/ folder. But because of the integration with Xoops, it seems it now uses the XOOPS system for language definition, meaning the actual language definitons are made in files in the /spaw/language/ folder.
By replacing the 'nl' argument with an empty string ('') in the line I mentioned earlier, the Spaw class refers to the default language set in the spaw_control.config.php file. In my case this was 'en'. This solved the problem.
It might have something to do with XOOPS obligating the use of the XOOPS languaging system? I don't have a clue why all of a sudden it stopped working, when it had been working for weeks... Maybe someone else upgraded some XOOPS files.
Hope this helps!
Thanx again!