It usually pops up if you've saved your mission including an addon, and later changed your mind and deleted and uninstalled that addon... What I think has happened is that the addonname is still listed in the requiered addons list in your mission.sqm, delete that addonname an u sould not get any errors (note that you have to delete the addonname from all the different sections e.g. Intro, Outro, mission...).
Also "No entry 'config.bin/cfg weapons.
sebmg50" sounds like the addon is a 50 caliber seb nam (seb nampack 1 or 2) machinegun, deleted\uninstalled any of those lately?!?
EditOr maby I'm just confusing this with one of the other error messages (Hundreds of em) that plagues my comp every day ;D