755 permissions are OK as long as linux user is the same as user running your webserver.

I think cusomers and users are not cached, so if you create user or register a customer you should see thouse.

I believe there is no connection between wishlist and the stock. Back to sock notification would be a great plugin though. Please PM to me if you want it to be created.

Welcome to Abantecart gear259!

You might have one of below two issues:

1. If you have multiple stores setup, make sure you have a correct store selected in the admin. There is a store selector in the top right corner.
Customers, products, etc are created per store.

3. Abantecart cache is not cleared.
Try to clear the cache
https://abantecart.atlassian.net/wiki/spaces/AD/pages/14483758/Cache
If this does not help, you might have permission issue on the files/directories
folder files under .../system/cache/... should be writable for a user running a web server.

Note. It is not recommended to make changes in database (phpmyadmin) directly, unless you know what you doing.

The question is to you. Did you or any one on your team do any upgrade or changes? What is a version of your AbanteCart?

Based on my experience, there is something changed on your configuraton or hosting side.

There must be something on the AbanteCart or webserver log. Check and share the errors right after login attemnts.

Your FTP issues it not related/connected to Mysql error.

I think your error is self explanatory. you need to repare the table.
https://dev.mysql.com/doc/refman/5.7/en/rebuilding-tables.html

mysqlcheck --repair --databases [db name]or
mysqlcheck --repair --all-databases

I can do this for you if you want. Please PM.

504 error with Gateway time out can be many things. The majority of problems can be on the server side and necessarily a bug in the code.

 Does it happen one time in the same page/request or consistently?

Thank you llegrand for the comment. I posted to quickly. I corrected the post.