16
Bugger! Why didn't I spot that one?
I think, I had the same message on my local installation of Mambo on my Linux server - simply because I did not have XML extensions on the server.
Check with your host if this is possible.
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The AgendaX notices are just that - notices. It doesn't mean that the code is broken, but the author has used a variable or index of an array, which does not necessarily have a value and hence, these notices that the variable/array index is empty is given.
Unless the variable must have a value for the page to work, it is nothing to worry about (but should be avoided when writing modules)