Fixed on your site and in the next release. This occurred in the following condition:
1. A Manage Synonyms tab is open, and at least one synonym is listed under the existing synonyms there.
2. One of the existing synonyms is deleted in another browser tab or by another user.
3. The user with the Manage Synonyms tab open saves the tab.
4. Since one of the existing synonyms on the tab no longer exists, this error is thrown.
The correct behavior in this case is for the synonym to be recreated without throwing an error.
In this case, this resulted in the prefix selection not being honored, and no prefix being used when the synonym was recreated.
Additionally I noticed that when creating a new synonym on a page that already has at least one synonym, the prefix selection for the new synonym is not honored. However, it should still be possible to rename the synonym to include the prefix afterwards. This is also fixed.