Since a few days logging in to our Magento (1.8.1) backend is impossible - but only from one specific IP address, the IP address that used to be set at System -> Configuration -> Advanced -> Developer -> Developer Client Restrictions.
The issue at hand is that the login seems to be successful, yet I receive a blank admin page instead of the dashboard.
While trying to troubleshoot this issue I have of course changed this IP address to another IP address (220.127.116.11), cleared sessions and caches, even rebooted our server, but no luck.
Still receiving a blank page.
I activated all of the possible logging / developer options hoping this would give me a clue of what's wrong, maybe even on infamous fatal error, but no. Just a white screen and no suspicious or clueful entries in nginx, php or magento logs.
What makes it more strange is that this problem happens on multiple computers/browsers behind one specific (and fixed) IP address, yet if - for instance - I connect one of these computers to a 3G Hotspot (and as such connect using a dynamic IP address) I am able to successfully login to our backend. Now if I disconnect from the hotspot and on to the regular wifi (and as such continue using the fixed / problematic IP address) it works perfectly. I can navigate all pages of the backend -including the dashboard- without any issues.
Once logged off or timed out the issue re-appears.
Someone somewhere any clue on why this is happening and how to solve this?