This is such a frustrating error. It's almost always due to some corruption caused by interruption from the client to the server on exit/save. Since you've already tried deleting the SimCityUserData folder, it's probably not a problem on your end, but on the server side, unfortunately. People have not seemed to have a lot of luck recovering from this. Back when the game was new and it was happening all the time, I believe they asked for the information you provided and went through and tried to troubleshoot or roll back cities where it was happening. I don't think they are doing the same now, though.
There are only a couple of things that I've seen reports of recovering from this.
- Go into a city in the region that does load, then go to region view and see if you can load the problem city from there.
- Run SimCityRecovery.exe Default location is \Program Files (x86)\Origin Games\SimCity\SimCityRecovery
- Delete the SimCityUserData folder and load the game again. (Program Files (x86)\Origin Games\SimCity\SimCityUserData) - I know you've already done this, but in case someone with the same problem is reading the answer, I'm including it in the list
- Do a Complete Uninstall of Origin, then reinstall
The only other thing that I have seen reported success with is to move *.mfs file associated with the game. The problem is that this will remove regional progress - so if this is your only city in the region, it might be enough for you, but if your city has a lot of academy or university research done, the region will not see it nor be able to use unlocks from any of it. You can use this trick to transfer all of the money out of the city to your other cities, though.
If you want to experiment and see if this helps:
Close SimCity and Origin.
Go to User_Name\Documents\SimCity\Games and find the folder associated with the city.*
The file you're looking for is named SLDelta####.mfs (where # = numbers). Move the file somewhere you can find it again, such as your desktop.
Open SimCity and try to get back into your "unable to load" city
If it works, you will have lost all regional progress for anything in that city. Either transfer all $$ from that city to another one, exit and restore the moved file and just leave the broken city, or you will have to go through all the research and unlocks in a different city in the region to unlock them again.
One idea that I've not seen anyone else post that they tried, would be to remove the SLDelta##.mfs files from all folders and all cities in that region. Not sure if it would work, but it seems to me that if the game is going to create a new one on login, it's possible that the game could then "add" the regional unlocks from every city. You could give that option a try. (be sure to move, not delete, the files, so you can restore them if it hoses the whole region) If it works, please reply and let us know. This has been a problem for many users over the years, and it would be wonderful to have a solid, known fix to pass along. Good luck!
*If you have many different nested folders, it's difficult to tell which folder it is for the city. Try going by the date modified to find the correct one.
If none of the above works, you could try calling support to see if they can roll back your city. Otherwise, you may have an unrecoverable city. Good luck!