Scheduled tasks not saving changes [Coldfusion2021]

Please share the entire stacktrace of the error. Review ColdFusion's logs and you should find the stacktrace. 

On second thoughts, the message "Invalid extension of the file name. Valid extensions are :log,txt" might just be validation feedback of the scheduled-task interface in the Administrator. In which case, it will not percolate down to the logs.

Suggestions:

1.  Open /lib/neo-cron.xml in an editor and ensure that the "publish" element for the sheduled task is
   

   <var name='publish'>
   <boolean value='false'/>
   </var>​

2.  Temporarily comment out the line 
   

    writelog( file="myfile", text="starting myfile.cfm" );​

   
   and see what happens.
3.  Open /lib/pathfilter.json in an editor. Check whether the attribute "schedulerexecutionpaths" contains a file path that ends in an extension other than LOG or TXT. If it does, that might be the cause of the error message. 
   In any case, change the setting to
   

   "schedulerexecutionpaths": "opt/coldfusion2021/cfusion/logs/**"​

   
   Restart ColdFusion, then see if you can edit the scheduled task without error. 

There are a couple issues here. First, the fact that you get that error is because the task you're 

editing is trying to "publish its output" to a file. See the settings within the task in the cf admin. 

And the fact that you're getting the error about its file extension suggests that your cf2021 had not been updated in quite a while. A previous release forced that file to have one of those two extensions. 

So your first step is to edit that setting to name a different file extension. 

But then you will hit a NEW issue that's specific to this latest May 2025 update, which is update 20 for cf2021. It implements yet another security improvement that requires specifically whirelisting the path to that file in the new pathfilter.jsonnfile. For more, see the blue box under the code sample offered in the section labeled "Pathfilter changes".

Finally, folks applying these May 2025 cf updates should also see a related point BEFORE doing the update, as covered in a point in the "known issues" section, in the bullet starting with the sentence, "After installing the update, the scheduled tasks configured with the Save output to a file option enabled may be removed from the neo-cron.xml configuration file...."

[As a clarification, when I FIRST replied I thought you'd hit that second or third problem, the third of which is so lamentable when it happens unexpectedly. I've now corrected this reply to address your issue as my first point here.]