Insufficient Permissions for wp-admin

Hiya. Hopefully someone can shed some light on my problem

I recently upgraded to wp 4.4.1, updated all my plugins etc. and realised that some of my pages were giving me a white screen of death. I also found that I couldn’t access the wp-admin dashboard without getting a “You Do Not Have Sufficient Permissions To Access This Site”.
It seems the white screen is down to w3total cache. But the wp-admin problem still persists.

I’ve deactivated all plugins, the theme, I’ve reinstalled wordpress, I’ve downgraded back to wordpress 4.3.2, I’ve checked all the usual suspects like curly brackets in the database, prefixes etc.
Technical support at Ipage have worked on it for the last few days and pretty much given up! They said they couldn’t track down the problem.
After they gave up I tried wp-debug and found the following error:

Warning: mysqli_real_connect(): The server requested authentication method unknown to the client [mysql_old_password] in (website)/wp-includes/wp-db.php on line 1488 Warning: mysqli_real_connect(): (HY000/2054): The server requested authentication method unknown to the client in website/wp-includes/wp-db.php on line 1488 Deprecated: mysql_connect(): The mysql extension is deprecated and will be removed in the future: use mysqli or PDO instead in (website)/wp-includes/wp-db.php on line 1518

They told me to downgrade back to php 5.3 (I was on 5.5). Which I did and I downgraded to wordpress 4.3.2 which solved all wp-debug issues. But I STILL CAN’T LOG INTO WP-ADMIN!! grr.

I can get into all the admin pages eg. wp-admin/plugins.php by going to them directly. It’s only the actual wp-admin page I can’t access directly. So I can get round it and the website is working but I don’t like the fact I have a major problem at my wordpress core. Surely it’s symptomatic of something more serious. I’ve looked through the forum at almost every other instance of the insufficient permissions issue and nothing has worked. Any ideas?