I found a solution to a problem that does not require deleting your whole save data!
If you have the Alert while attempting to join GTA Online that says:
Your save data could not be loaded from the Rockstar cloud servers at this time.

The solution is to delete 1 file by the name of cloudsavedata.dat which is located at
C:\Users\<Your Windows ID>\Documents\Rockstar Games\GTA V\Profiles\<Your ID number>\
After that the Social Club window will ask how you want to save your game - on a cloud or just locally.
Don't worry, your Single Player save will be intact and you should be ready to go GTA Online with no problem.

Please let me know if this worked in the comment section. It did for me!
If it does for you too, rate this 5 stars please :-)

Have a nice day and GL HF !