Mantis#1910. Thank you kindly, HomerHorwitz for a patch that:

In case you run the server with   mono --debug OpenSim.exe
line numbers are reported as <filename>:<linenumber>, so no 
"at line" is found. That led to an exception, which is caught 
since r5766. The attached patch fixes this for mono;

1 files changed, 5 insertions, 1 deletions

diff --git a/OpenSim/Region/ScriptEngine/Common/ScriptEngineBase/EventQueueThreadClass.cs b/OpenSim/Region/ScriptEngine/Common/ScriptEngineBase/EventQueueThreadClass.cs
index 4969df0..43905a7 100644
--- a/OpenSim/Region/ScriptEngine/Common/ScriptEngineBase/EventQueueThreadClass.cs
+++ b/OpenSim/Region/ScriptEngine/Common/ScriptEngineBase/EventQueueThreadClass.cs
@@ -319,7 +319,11 @@ namespace OpenSim.Region.ScriptEngine.Common.ScriptEngineBase
                                                 // way, need to skip the word "line " in the substring
                                                 try
                                                 {
-                                                    line = " at line " + Convert.ToInt32(t.Substring(colon + 6)).ToString();
+                                                    // ...if it is there. With mono --debug OpenSim.exe,
+                                                    // you'll get the error in the format filename:linenumber
+                                                    if(colon + 6 < t.Length && t.Substring(colon + 1, 5).Equals("line ")) colon += 6;
+                                                    else ++colon; // else only skip the colon
+                                                    line = " at line " + Convert.ToInt32(t.Substring(colon)).ToString();
                                                 }
                                                 catch (ArgumentOutOfRangeException e2)
                                                 {