Just had a family member with exactly the same problem :

Booked 5 different flexible reservations at the Sheraton Dubai Creek on Dec 8.
None of the reservations are visible on their account, and gave the "We're experiencing a technical issue. Sorry for the inconvenience. Please try again later." error when using the "Find my reservation" function online.
The "Find my reservation" function on the app, however, did work and allowed the reservations to be 'reattached' to the account.

Looks very much like a non-user-specific issue that's affected all reservations at this hotel that were made in a particular time-frame.