PHP Catchable fatal error: Object of class WP_Customize_Manager

Hi,

Since upgrading to the latest WP version, im having issues with the customiser, with the following symptoms:

1. The user cannot navigate between pages
2. The browser title bar never changes from “Customize: Loading…”
3. The widgets panel won’t show any widgets position available
4. Occasionally getting the following error in log:

[02-May-2016 11:10:49 UTC] PHP Catchable fatal error: Object of class WP_Customize_Manager could not be converted to string in /srv/wordpress/wordpress/wp-includes/query.php on line 4518

Even weirder, when widget positions do show, if Im on content page and I add a widget or make any change to an existent widget, the customiser will reload, briefly show the homepage and then go back to the page I was seeing. Sometimes it appears to be stuck halfway through, and it shows what appears to be a stuck CSS opacity transition between the two pages! Really weird stuff.

I first detected this with my theme applied, but tweentyfifteen shows the exact same issues.

I should point out that im not 100% certain these started right after the upgrade, and it also worths mentioning that very occasionally the symptoms go aways and it works as expected.

Have anyone seen this before? Not really sure where to look next…