The OS is 10.3.9 on a sawtooth G4 because I'm still not ready to give up my SE/30 file server.
I'm still running OS 9 on some machines. I have scripts written in System 7 that still run on OS 9 and 10 and classic. Your setup shouldn't cause any issues for applescript.
Script Editor says it's version 2.0, 11 July 2007 05:02.
Shouldn't be a factor.
I have several very simple "applications" that were prepared long ago to do simple things. Tell Finder to update frontmost window, tell Safari to empty cache, Execute some shell script that mounts my SE/30 disks on login (10.1 needed that.). Some of them just stopped working! No error message. Nothing. Double click, open from Finder's menu, open from a BBEdit worksheet. All the same. I have no idea, until I try using one, which will work and which won't.
May not be a problem with the scripts. You may want to run disk utility, fix permissions, even reinstall your system.
The fix is to reopen the files in Script Editor and re-save them as applications while checking the box named "save as run only".
DON'T DO THAT! That won't "fix" anything and will "break" your ability to edit these scripts. That's a one-way ticket to nowhere.
I don't remember that box from earlier days and the scripts don't contain an "on run" block. They're just a few lines and some don't have any tell blocks in them. Some use system events scripting. The compile option in Script Editor doesn't complain.
A run block is entirely optional and not a factor.
Does "run only" imply some kind of protection against changes? It doesn't seem to but I am the owner.
Run only protects the script even from its owner. DON'T DO THAT!
Is it possible that an Apple security update did something?
Don't think so, I've installed every security update that's come out and haven't noticed a problem with new or old scripts
Is it possible that something timed out after 4 years?
Maybe if a script has been running non-stop for that long, but with normal every day use, no.
Did the underlying AppleScript rules change?
For the most part, Apple has been pretty good about not breaking old scripts with updates. This sounds more like a finder/file system problem.
Are AppleScript apps really APPL's or are they really run by Script Editor which seems to be fairly new?
They are applets, that have no connection to Script Editor at runtime. They do, however, rely on AppleScript and Scripting Additions being properly installed.
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--> The best programming tool is a soldering iron <--
Or a hammer, for optimization.