First task would be to do a repair installation in the launcher, or manually download the updates.
If you are getting this error your MBAssets3.pk3 is >3 updates out of date.
Oh I didn't know that, thanks a lot for the quick help!If you need the CMP, MGMP, uM map packs then you can just do "complete installation" in the launcher or download the full release. Those have been updated to be compatible and are part of the main release now.