I'm just a user, like you - no connection to Roku.
This is an odd issue, and I am just curious, but a few details may help you and others.
First, can you try logging into YouTube with another ID?
Do you have parental control on?
I assume that YouTube works fine on other devices in the same home, same login ID - can you please confirm?
Once in a blue moon, I may access a channel, and have to "wait" - I just hit "Home" and then re-connecting usually solves the problem.
I know it is a pain, but can you delete just YouTube and try it with ONLY activation, NO subsciption?
That's all I can think of...
All the best, but let me know. Thanks