One Reddit user recently reported that their YouTube player was blocked after using an ad-blocking browser extension. YouTube had previously announced this measure in May, stating that if an ad blocker was detected, a notification would appear on the player and users would only be allowed to play up to three videos. This system has now been implemented.
Source: Reddit (1), Reddit (2) via 9to5google
TLDR: YouTube is now blocking users from playing videos if they have an ad blocker installed, limiting them to only three videos.