Hi Juan,
I've never seen that one before, but by looking at the code I wonder if its due to a race condition when initializing the "checkpoints" variable on the request scope. The variable "__buglog_checkpoints__" is only set when is requested, so I imagine it may be possible for 2 requests to step on each other.
If you continue having problems with it and you are not using the checkpoints feature, then you can update your client CFC to remove the checkpoints section from the bug report (See method composeFullMessage() )
Thanks,
Oscar