6.1.1 Error creating any new item - by: Pirlitescu

Hello,

I'm getting this error: ERROR: An error occurred on [date] at [time], every time I try to add a new item, regardless the topic (users, projects, documents etc).
When opening the log file, the error message is "A non well formed numeric value encountered".

This is a brand new installation and no errors / warnings were found during setup. I've kept the default english localization.
The site runs on a Mac, using MAMP (zendengine) server and a standard MySql db server.

Any hint?

Thank you