When creating a keyword dictionary from the Microsoft 365 compliance center (Data Classification -> Sensitive info types -> Create sensitive info type -> Create pattern -> Add primary element -> Keyword dictionary), any keyword in your dictionary which is longer than ~500 characters will trigger the page to hang for several minutes, and then give a generic error saying to try again later or contact support if the problem persists.
After extensive debugging over a call with support, I discovered that the real problem is the existence of any keyword over 500 characters long.
This isn't documented anywhere and there is no way for anyone to figure this out without manually debugging as I have done.
Would it be possible to create a new error, in this case, which explicitly tells the user that the keywords they've included are too long?
Without such an error, there is no way that anyone would know what they're doing wrong.