Well, WP ERP has sure become a mess. Installing the latest WP ERP free version from the WordPress repository and WP ERP Pro 1.2.6 works fine, as long as you don't install any modules. LOL
Unfortunately, it looks like the changes that cause all previous versions of external modules to break the site, throwing an "Nonce validation failed" error, started in WP ERP free version 1.10.6.
Keeping the free version at 1.10.5 seems to work with the external modules and WP ERP Pro 1.2.6.
I guess until someone gets access to the new versions of the modules, this will have to do.