Less than two weeks after YouTube experienced an hour-plus-long outage, the video service was sporadically returning error messages Monday for users trying to play back videos.

Error messages included "This video requires payment to watch" -- for videos that are free to watch -- and "We are experiencing problems with our servers. Please try again later." Sometimes the same videos played properly if the site was refreshed.

The website DownDetector.com showed a spike in users' error-message reports for YouTube starting a little after 2 p.m. ET, with most reporting trouble watching videos. According to DownDetector.com's map of error reports, the problems appeared to be mainly affecting users on the East Coast of the U.S.

In replies to YouTube users asking about the error messages, the Team YouTube account on Twitter told users that it was "currently investigating" after seeing similar reports.

Reps for YouTube didn't immediately respond to a request for more information.

On Oct. 16, YouTube had global access problems for around 90 minutes, with issues that also affected the YouTube TV and YouTube Music services. The company apologized and issued credits to YouTube TV subscribers, but did not fully explain the cause of the outage.