In lurker's RSRDC, he has [Unit 44] Manus Island going active on May 1, 1944. Save your game, and exit. Look through your other mods, and see if they have a "Data / Campaigns / Campaign / US_NavalBases.mis file in them that is overwriting lurkers, and back those mods off. Whichever mod(s) has a US_NavalBases.mis file in it, remove that file from the mod, then re-activate it and any other mods you had after that. Now start SH4 again, and see if you can load that Save. If not, do NOT Save, but exit the game and undo those mod changes again. You'll have to put that US_NavalBases.mis file back in, but use WinMerge to reconcile the differences with the bases, or using a text editor, edit the offending US_NavalBases.mis file to either include Manus, or alter its date for Manus, if it has one in it. Use of WinMerge might involve a ~massive~ number of [Unit ###] changes, and you might be better of altering the one, or adding one at the end of the file, using the next available [Unit #] after the current last entry...
