I have used Vassal and Command and COlors Ancients modules for some time.
I recently had a need to reinstall, but now the only module that will load is the main module. All of the other modules and map modules give the error that they are not valid Vassal modules.
I only see two actual modules on the site:
C&CAncientsv3.vmod
CCAncients_d10.vmod
The rest are extensions. It sounds like you’re trying to open an extension as a module. Right click on the module in the module manager window and select “Add Extension…”