I am in the midst of working out my Apollo13 procedure for my JMS site platform.
Upgrading from J! 3.6.5 to 3.9.10. PHP 7.1.30, mySQL 5.6.4.

When I go to several of the slave sites Global Configuration page in the administrator area, I am getting the following error:

Error decoding JSON data: Syntax error

This is not affecting all slave sites, and it is not affecting the Master site's Global Configuration page.

I found the solution:

Solved by editing the following:

- In phpMyAdmin, open the Table: ihnj3_extensions,
Then open the com_config extension and at the end of the parameters change the following text:

"filter_attributes":""}},}},}},}},}}}

to:

"filter_attributes":""}}}

Deleting a comma and leaving only three curly brackets.
Global config page now displays.

------------------- This is how I figured out the above solution:

I uploaded this script to the server and went to it. (Backed up the site database first.) json-db-check.php from:
github.com/robwent/joomla-json-db-check

Visited it in a browser and it revealed the json errors.

Then I opened the problem slave database in phpMyAdmin and viewed the table and row indicated which was in this case, #__extensions and the com_config table.
I copied the bad parameters and saved as a text file, then opened the database for a slave without the JSON error issue and opened the com_config table and copied its parameters, saving it as a text file.

Then I used a file compare app to compare the two, and modified the bad parameters as needed to match. It worked.

Don't forget to delete the json-db-check.php script from the server.

I don't now how you got this joomla core table corrupted but thank you for the procedure to explain how you solved it.

Regarding the cause of the corruption, I read somewhere that it might have something to do with a JUpdate in the past. Many of the sites in this multisite were imported into the system. I solved it during the Apollo 13 process.