This issue popped up earlier this year for a few people and Zwift recommended a support ticket (see here). No solution was mentioned in forums but I’m picking its a connectivity issue in Zwift land somewhere tat it didnt recover from.
I’m a ride away from the 50k as well and last night I accidentally hit the trash can rather than save button and had a coronary until I realised it had a “are you really sure …” followup question, so totally get your anxiety around this level. Never seen the Zwift team load a users fit file after a problem but you can load into Strava (doesnt help you with the 50k achievement sorry).
Log a support ticket and ask them nicely to load the file fit file while they are at it. Good luck.