Cannot login to frontend after server migration

[Peakoverload]

My webhost recently migrated my site from Cpanel to Site Tools. Ever since then I've had problems with logins.

Immediately after the migration I couldn't access the backend at all. When I tried I got a message saying something like the file could not be found.

This turned out to be that I have a php.ini file in my administrator folder and the open_basedir path was no longer correct under Site Tools. Removing the php.ini file resolved this and I can login the backend.

However I can't seem to login in the front end. At first I thought it was because of a php.ini file I have in the root, so I removed that but it made no difference. I then remembered that at the same time of the migration I also decided to switch the version of PHP from 7.3.27 to 7.4.15. I switched back to 7.3.27 and I was able to login the front end and posted a message in the forum on the website. Today, I can't login again and nobody can login!

The backend is fine but when you try to login the front end, all that happens is you get taken back to the home page. The backend doesn't register that you've tried to login and I can't think what else to check. Any ideas?

UPDATE: Well this is starting to freak me out. Prior to posting this I could not login. After posting this I logged into the backend and thought I'd see if any errors etc get reported. I turned on debugging and tried logging in the front end and BINGO I'm in!??? Turned off Debugging and I can still log in.

I'm wondering if this is a cache problem. I use a cache cleaner on the website and when I enabled debugging it automatically cleared the cache. Perhaps that's why I could then login. If that's true, then why couldn't I login earlier when I'd already cleared the cache yesterday?

[Per Yngve Berg]

Please use the Forum Post Assistant so we can review your server Configuration.

Using open Base in hosting is so old fashioned.

[AMurray]

Assuming you are with SiteGround and had the migration of cPanel to SiteTools.

I also had sites migrated but no issues with front-end logins or any issues with my sites, period.

Could be caching - do you have Sitegrounds's "Super Cacher" or third party CDN such as CloudFlare etc enabled?

[brendanhedges]

I too agree, sounds like a cache issue someplace. I've also had weird login issues in the past caused by my .htaccess file. So it's always worth renaming it temporarily to take it out of the equation while you search for a fix.

[richardh9935]

"Peakoverload" have you resolved this yet? If so, how?
(I have a similar problem.)