503 Service Unavailable when I try to login

Hi
This site has been working fine for years until we try to update something like yootheme or Joomla as we did today with Joomla 3.10.12 out
We get the whole front of the site showing a 503 error but administrator fine.
I realised that swapping template would remove the error and so I suspected a module. 
We worked through all the modules publishing one by one until we  got to the culprit and once we switched that off we could switch all the others back on and no issue. But the issue is with the login module.
I have updated from 2.8.0 build in January to the one today and now we don't get the site broken if we go to the home page or any other, but if we try to log in we get the 503
www.publishingtrainingcentre.co.uk/cb-login

Any ideas. As I say, this has been sweet as a nut for many years and recently this.

www.publishingtrainingcentre.co.uk/index.php?option=com_comprofiler&view=login = 503

In Joomla global configuration enable debug mode and maximum error reporting then try and see if any errors output. If you see no error output check your server error log.

Doesn't seam necessarily related to the login module, but is due to one of your CB plugins that's being loaded in. Even CB > Plugin Management causes a 503. Did you install or update any 3rd party CB plugins recently? Also contact your host and have them check mod_security logs as sometimes it's just a false positive causing this.

Hi Krileon
Not installed or updated any 3rd party plugins recently. As I say its been really stable for years.
I checked with my server guys yesterday before messaging and they could only point to CB login as the cause which I had already concluded.

Error  on max gives nothing and the logs show the following every time you try to log in

Hi Krileon
Not installed or updated any 3rd party plugins recently. As I say its been really stable for years.
I checked with my server guys yesterday before messaging and they could only point to CB login as the cause which I had already concluded.

Error  on max gives nothing and the logs show the following every time you try to log in


Just noticed this when in 
www.publishingtrainingcentre.co.uk/administrator/index.php?option=com_comprofiler

Notice: A non well formed numeric value encountered in /var/www/vhosts/publishingtrainingcentre.co.uk/httpdocs/components/com_comprofiler/plugin/user/plug_cbpaidsubscriptions/cbpaidsubscriptions.php on line 25

Fixed, it was an issue with a paid subscriptions plugin.
Repurchasing and updating the paid subscriptions plugins removed the error and login seems fine now