Hey @stevenp
thanks for giving more information.
The installed Agent version is a 1.6.0p33 but from the other information you provided shows that your Checkmk Server is still running on 1.2.6p12.
Your problem is that your old Checkmk Server (1.2.6p12.) can’t deal with the newer Agent Output (1.6.0p33) and thus crash.
You can provide the crash report if you want but you should really thinking about updating your Checkmk Server asap. 
Regards
Norm