The way I solved the problem: “There was an error saving the message to Sent. Retry?” is:
1) Create a new map in the mail account folder and call it something like ‘Sent emails’
2) Go to ‘settings’, ‘copies & folders’
3) In the first section : ‘When sending messages, automatically’ – [x] place a copy in:
select => other
4) Select the folder you made in step 1)
5) Click OK
6) Delete the original folder ‘Sent’
Now all sent messages should be stored in the new map ‘Sent emails’ without problem
Please let me know if this solution helped you or if you have found an other solution to the Thunderbird “There was an error saving the message to Sent. Retry?” bug.