sorry for the late reply here - an empty sidebar usually indicates a database schema issue for new users, but it’s hard to tell without additional details.
Do you see any error messages in your mattermost.log file and do you have access to the database? If so it would be interesting to see the differences in the tables for a new and an existing user.
SELECT * FROM user where name IN ('workinguser', 'notworkinguser');
Remember the userid of these user accounts and use it in the next few queries:
SELECT * FROM preferences WHERE userid IN ('userid1', 'userid2');
SELECT * FROM sidebarcategories WHERE userid IN ('userid1', 'userid2');
SELECT * FROM sidebarchannels WHERE userid IN ('userid1', 'userid2');