Hi @stephbat and welcome to the restic community 
restic does allow concurrent access for multiple backups to the same repository.
Some restic commands set an exclusive lock. You can read more about that here: References — restic 0.16.3 documentation
You won’t run into a restic internal issue with 25 writes to the repository.
Is it possible to check if there’s a lock on the repository before launching the backup ?
Yes see the restic help section for this.
Now, it would be really helpful to see the entire command you ran without any redactions - obviously we don’t want to see your password for the repo.
Also have you utilized the forum search before starting a new thread?
I did. Search results for 'The specified key does not exist' - restic forum.
And I came across this post here: Load snapshot: The specified key does not exist and FindLatestSnapshot error
Possibly there are other threads with people who ran into the same problem.
If you don’t find any solution to your issue, definitely come back here and share what you have learned so far.
Also, please make sure you try out a new version of restic. Yours is pretty old as the latest stable release is 0.11.1. So make sure you go to GitHub and get the latest release binary for your platform.