We are running multiple Magento installations on seperate cPanel accounts on the same server.
One of the shops has started to experience a strange error after upgrading from Magento 188.8.131.52 til 184.108.40.206.
Customers are reporting that when they try to log in to their customer account at domain.tld/customer/account/login, then they are redirected back to the login page.
Through testing, we tried to log in as a customer and experienced the same behaviour. Through the network tab in Chrome Dev Tools we can see that we are posting to domain.tld/customer/account/loginPost and recieve a 302 redirect to domain.tld/customer/account and this once again returns a 302 redirect to domain.tld/customer/account/login
Our test customer had something in the shopping cart last time they tried to log in. We can see this when we tried to log in as our test customer. Before login, the shopping cart was empty, but after trying to log in, we see 25 items in the shopping cart, so some kind of login is done.
Login to the admin area works fine.
When trying to click the link to "My Account" or visit domain.tld/customer/account/ manually, we see a 302 redirect to domain.tld/customer/account/login/
The 3rd party design does not seem to contain any changes or overrides for the default login pages or customer account pages.
We have tried to clear the session folder in public_html/var/session and also tried to disable 3rd party modules and leaving only the 3rd party theme/design and core Magento modules enabled. This did also not change anything.
We have tried to verify that the login actually happens by disabling the automatic redirect to the customer account page. This results in the customer being redirected to the page they are coming from and again we see the 25 items in the shopping cart, but are still unable to visit the account page.
Are there any good ideas out there on how to further troubleshoot or maybe solve this issue?