We had someone report something similar to us via email. Though the error was being thrown from DarkRP, a ULib issue may have been involved too.
However, we fixed the ULib issue and I see that you're using the latest Git version of ULib, so the DarkRP error is probably the culprit. I'd recommend reporting the error to FPtje
on his issues page, he's normally pretty good at responding quickly. Make sure you include the stacktrace for the error from your server console.