[Shedi]

1. Cookie Settings Issue
Incorrect cookie settings can prevent logins.

Solution:
Go to Admin CP > Configuration > Site Settings > Cookies and Sessions.
Ensure the Cookie Domain is correct:
If your forum URL is https://example.com, the cookie domain should be .example.com
If it's https://www.example.com, set it as .example.com
Set Cookie Path to /
Save changes and clear your browser cookies, then try logging in.
2. Incorrect Board URL
If the board URL is incorrect, login sessions may break.

Solution:
Go to Admin CP > Configuration > Site Settings > Board URL.
Ensure it matches your actual forum URL (e.g., https://example.com/forum/).
3. Cache Issues
Sometimes, the forum cache might be outdated.

Solution:
Go to Admin CP > Tools & Maintenance > Cache Manager.
Rebuild the settings cache.
4. Plugin or Theme Conflicts
A faulty plugin or theme modification might be blocking logins.

Solution:
Disable all plugins from Admin CP > Configuration > Plugins.
If you can't access Admin CP, rename the /inc/plugins/ folder via FTP to disable all plugins.
Try switching to the default MyBB theme.
5. File Permissions Issue
If session files can't be written, logins may fail.

Solution:
Ensure that the /tmp/ folder (if sessions are stored there) or cache and uploads directories are writable (CHMOD 777 or 755 depending on your server).
6. Browser or Device Issues
Some users may experience login problems due to browser cache or extensions.

Solution:
Ask users to clear their browser cache and cookies.
Try logging in using Incognito Mode or a different browser/device.
7. Server-Side Issues
Your hosting provider may have restrictions affecting sessions or cookies.

Solution:
Check for errors in Admin CP > Tools & Maintenance > System Error Log.
If using Cloudflare, disable "Rocket Loader" or enable "Bypass Cache on Cookie".
If the issue persists, let me know what error users are getting when trying to log in!