A quick forum or Google search didn't reveal any obvious fix so here it goes.
It's been a really long time since I've played FSX (SE version) so today when I tried to play and the scenario wouldn't load I decided to reinstall PMDG, since this problem appeared every time I used the 738 NG.
I uninstalled PMDG, manually deleted the Operation Center folder on the Program Files and proceeded to download the latest installer. Everything went okay with the installation, except the Operations Center Updater, which every time I run it shows the following error:
"Product Module 737NGX XSE.dll" has failed to download after 3 attempts. Please try again.
I'll try restoring the Operation Center directory from the previous install, see if that fixes anything.