why does outlook calendar tell me "cannot save item"? Is it full

[Peter in Brussels]

outlook 2003 - plenty of items already entered in my calendar - suddenly I
can add no more - each attempt to save a new entry generates a "cannot save
item" message

does outlook reach saturation point beyond which it can take no more?

I have archived a bunch of stuff from it in the hope of making space, but
this hasn't worked.

Any good ideas?

thanks

Peter in Brussels