I was going to post about this but wasn’t sure where to put it. This is affecting a number of users across Steam currently and is possibly due to a recent Steam update. You’ll probably also find that youtube videos won’t work in the Steam overlay web browser either. As far as I can find no fix has been figured out as of yet but it seems to be affecting other users too. I imagine not many people have picked up on it yet because the volume of people who use the ingame browser for youtube/the amount of people who play TU isn’t a big part of the Steam userbase. I believe it also affects Steam’s broadcasting feature which hopefully means Valve will be quick to fix it.
Here’s a link to the largest current Steam thread on the issue: https://steamcommunity.com/discussions/forum/1/558748653728030232/#c135512625256347204