Just ruling a possibility.....
Is there any chance that the computer on which you had this timer script installed hibernated?
This might be a bug to report in Report an Issue.
Nope, it does not go into hiberation. It does have a different user log in sometimes, while the original FileMaker user hosting the database remains also logged in, but this user switch does not affect the OnTimer script in the first 24 hours.
Is the script performed by the timer in the same file as that of the timer?
The reason that I ask is that the only ways I can think of for this error to even be possible short of a bug are:
A re-log in changed access permissions and the permissions deny access to the script performed by the timer
The script performed by the timer is in a different file and that file was closed at the time the timer ran down and tried to perform the script.
Both seem unlikely so I suggest that if you haven't figured this out, that you post an Issue Report documenting this as a possible bug over in the Report and Issue section of this Forum.