Adblocker nag screen
Project-GC will display a nag screen when it detects that the user is using an adblocker. This nag screen does not know whether the user is a paid user of Project-GC or not, so it will complain even for paid users (who don't get ads shown to them) who have an active adblocker. The solution here is to whitelist Project-GC in their adblocker which will deactivate the nag screen, but being paid members they still won't see any ads.
Other things interfering with the adblocker nag screen (and solutions)
Disconnect plugin: With the "Disconnect" plugin present in Firefox and ABP running, a user reported having to have to turn the "Disconnect" plugin off (thus allowing Google tracking) to the nag screen to not trigger.
Kaspersky antivirus: A user reported that he had to whitelist Project-GC in Kaspersky antivirus in order to not have the antivirus trigger the nag screen (Mac, Chrome).