Here's the history of that particular error.
During the private beta release of 6.6, bonienl pointed out that CA itself was popping up with that error on installation. Some testing, and a minor modification was made to CA's .plg file to avoid that.
A week or so ago, I implemented that change in every single one of my plugins.
Just to point out that if the error is being caused by user.scripts, then it it a harmless display error. On my system it does appear that user scripts is causing it. However, this is a real major pain to debug, as
I have to run my unRaid in GUI mode. In order to see the error, I have to reset everything to run in normal mode, which takes down my day-to-day system.
Doing a manual plugin install from the command prompt does not display the error.
The plugin works no problems, even with the error.
In order to test / fix everything I have to continually reboot the system to try and figure out where it's coming from, and then to test any changes to suppress it. This does not make for a very happy family.
IF it is user scripts (and depending upon which post you read it may or may not be), then it will eventually get fixed. But, bear in mind that it is a harmless display error as user scripts is functioning fine, and because of that the priority to fix it is not at the top of my list (mainly due to #4)