I have this problem every 4 months or so. Flush toilet and water level rises alarmingly in the pan and slowly drains away.
Just outside the house is an inspection pit that all the drains and toilets feed into. The outlet to the street sewer becomes blocked with toilet paper and the water level rises.
I remove the access cover and use drain rods to clear the blockage and clean up the pit with a pressure washer.
Perhaps 20 minutes work - wear rubber gloves.
Peccavi
October 2008