I have been experiencing this same exact sequence of (non) events since yesterday, and I know others have for longer. Very frustrating. My membership is due to renew next month. I hope this is fixed before then. We pay too much for such a basic thing to not work.
I've run many tournaments for my club without trouble before (for example, here is a previous tournament I made with the exact same settings), and the form hasn't visibly changed (and neither have I become less competent), but now:
It was initially rejecting the start date of 25th of December at midnight, telling me to add a valid time (error message repeated twice, i.e: please enter a valid time. please enter a valid time). After several tries, I got it to accept first a time by changing it to something other than midnight (which reduced it to just one iteration of the error message), and then a date by changing it to another date (so, 24th of December at 23:59), but now when clicking the final button to "Create", and confirming when the pop-up arrives about whether I really want to create it, the form just refreshes, with everything still present except for the custom trophies, which I then need to upload before trying again.
There appears to be no way out of this tournament non-creation loop; if I escape the error messages, then the tournament just doesn't get created, and sits there waiting for me to press "Create" (again).