Nokia n73 Low Memory

Press and hold the Menu Key (left of the 1 & 4) and you will see a list of open apps. You can shut them down one by one to free up ram.

This error refers to the amount of free space under "Phone Memory".
As in, the 44mb of built in storage.
Just any number of these, including:

• Call logs
• Messages (BIG bluetooth receives)
• Media stored on the phone, not on the card
• Old Calendar entries
• Browser bookmarks and saved pages
• Apps on the phone; reinstall them to the Memory Card!
• Unwanted contacts (harsh, I know...)

If this doesn't end the error, back up your stuffs then reset the phone using
*#7370#
Finally, this message differs entirely from "Memory low - close some apps." That one refers to low RAM limitations 

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only solution (that worked) was to re-load the firmware.