It looks like you may encounter this issue until you upgrade the clients to at least version 14.0.4.
From TS Gal, on a thread I'll reference:
The 307 error will show up with any version of FileMaker Pro prior to 14.0.4. Our Development and Testing department has discovered this occurs when modifying a field on a form in Form view that contains a sorted portal that shows either 0 or 1 row of related data. A workaround is to go to another record and then back again.
I have attached this thread as well as thread mentioned by "Markus Schneider" (Thank you!) to the original report. When more information becomes available, I will post again.
Solved (at least for us)
We had the same problem and it has been a challenge to resolve it.
First, it only happened to us on Windows (7.x or 8.x) users. It started when we enforced a policy of only installing the 64-bit versions of FIleMaker Pro. It happened to one user, then two, then four ... and it started to get out of control.
We rolled back about 100 FIleMaker Pro 14.x 64-bit installs to the 32-bit version and the problem stopped immediately.
Thanks to all for your replies.
I have developed a workaround, which is to add steps to the script running on the remote file to explicitly display and refresh the window for that file. I discovered that the error 307 would not occur if I first opened the remote file before running the script on the robot control file running locally on the robot. The debugger showed that the window for the remote file was remaining hidden during script execution on that file after being triggered from the robot control file. The remote file script now fully opens that files window and refreshes the window before the delete all step is executed, then closes that window when the script completes. No errors on two robots now using this technique. Something apparently "magical" about having a visible window for the remote file.
While this workaround technique is OK for a robot, where visibility of a remote file's window has no impact on the "user", it would not be satisfactory in many instances where there is a real human user looking at the screen, when it is desirable for the remote file window to remain hidden during script execution.
I haven't done enough research yet to conclude whether the error 307 affects only Delete steps, or whether other script step types may also be impacted by the underlying cause.