I'm trying to get some results in R, but after hours of searching I can't get past this error.

This is a little worrying: there is a cluster of values at the maximum level of Gepest_voelenRZ for Instructietaal Andere==0 (Google translate says this means "feeling bullied", "instruction language 'other'" in Dutch??) that are badly fitted. Try checking these data points to see if there's anything suspicious? (Poor model fits don't necessarily lead to numerical instability in model fitting, but they don't help ...)