Yup. Open the mission.sqm with Notepad (or similar) and remove all addon entries that are not used (anymore) from the addon list. Also make sure that all needed addons are listed there (can be quiet a pain in the a** to figure that out, because it's class names, not *.pbo names that are listed there).
class Mission
{
addOns[]=
{
"sebnam_vha",
"sebnam_vh",
"sebnam_wp",
"sebnam_nv",
"sebnam_us",
"editorupdate102",
"sebnam_vc",
"sebnam_ci",
"sebnam_a",
"sebnam_1a",
"sebnam_ls",
"Kolo",
"bis_resistance",
"sebnam_s",
"sebnam_m"
};
...
If your mission has (or once had) an in- or outro you should also check the *.sqm for those classes.
Example, no intro:
class Intro
{
randomSeed=4687363;
class Intel
{
};
};