<font face="Arial, Verdana">I have the same problem. It started in early april. Before that, it all worked perfectly for a long time. After 10-30 minutes of printing, RH crashes with</font>
<font face="Arial, Verdana">Unhandled Exception:</font>
<font face="Arial, Verdana">System.NullReferenceException: Object reference not set to an instance of an object</font>
<font face="Arial, Verdana"> at (wrapper unknown) System.Threading.Monitor:FastMonitorEnterV4 (object,bool&)</font>
<font face="Arial, Verdana"> at System.Threading.Timer+Scheduler.SchedulerThread () [0x00000] in <filename unknown>:0 </font>
<font face="Arial, Verdana"> at System.Threading.Thread.StartInternal () [0x00000] in <filename unknown>:0 </font>
<font face="Arial, Verdana">[ERROR] FATAL UNHANDLED EXCEPTION: System.NullReferenceException: Object reference not set to an instance of an object</font>
<font face="Arial, Verdana"> at (wrapper unknown) System.Threading.Monitor:FastMonitorEnterV4 (object,bool&)</font>
<font face="Arial, Verdana"> at System.Threading.Timer+Scheduler.SchedulerThread () [0x00000] in <filename unknown>:0 </font>
<font face="Arial, Verdana"> at System.Threading.Thread.StartInternal () [0x00000] in <filename unknown>:0 </font>
(Ubuntu 14.04LTS server, Linux 3.13.0-49-generic #83-Ubuntu SMP Fri Apr 10 20:11:33 UTC 2015 x86_64 x86_64 x86_64 GNU/Linux, Mono JIT compiler version 3.2.8 (Debian 3.2.8+dfsg-4ubuntu1.1, Repetier-Host 1.06)
Anyone who knows how to get around this problem, short of resorting to printing from SD-card? Many thanks!