The agent log had got too big!
65k is the limit.
ie all the calls to something like agentlog.logaction("text") were too big for the cache.
I could trap the error and it came up as error 4000 with error$="Notes error: Memory allocation request exceeded 65,000 bytes".
I vastly shortened all my debug mesgs and it solved the problem.
This could affect scheduled and web agents....
For other notes limits see: http://www.geniisoft.com/showcase.nsf/DominoLimits