Post Comment on Feed Intermittent Issue with Deactivated Author

[mention:31a4067d294a43c193ffc6a14711a9de:e9ed411860ed4f2ba0265705b8793d05] 
Maybe Appian uses cached user data, so sometimes it uses old (active) info.
Since when you are experiencing this error?

I would suggest(Future Handling), If your process allows user selection before posting, modify it to prompt the user to log in with a valid account before proceeding.

Hi, 

The issue you're experiencing with the "Post Comment to Feed Entry" using a deactivated user appears to be related to inconsistencies in how user status is handled within the system. It's possible that caching or delayed database transactions are allowing the deactivated user to occasionally perform actions. To resolve this, ensure that user status checks are consistently applied across your application and consider reviewing system logs for anomalies around these events.

quote "The author for the smart service must have permission to post a comment to the specified feed entry. Attempting to complete this task without the proper user rights generates an error message for the process administrator and pauses the process."

https://docs.appian.com/suite/help/24.1/Post_Comment_to_Feed_Entry_Smart_Service.html

I would recommend to check if the user you use for have access to all the feeds the usser should comment to. if its happening occasionly then there is a high chance that the user has not all the needed permissions aka group memberships or not maintain access groups. 
Could you figure out if a specific type of posts are effected- e.g. of the same application/appartment/orignal poster etc?