I’m sorry to see that you are experiencing this error. I have seen this before and I was able to clear the error by disabling a rule that was being triggered by ModSecurity on the server that I had BoldGrid installed on. Reviewing the Apache error logs helped to identify which error it was and then I simply disabled that rule in the ModSecurity settings. I recommend reaching out to the host of your server to see if they can identify that for you, if you need further assistance.
I hope this helps!
Sincerely,
Carlos D