Taxonomy autocomplete returns "connection reset" I have an autocomplete issue on an otherwise functional Drupal 8 site. I'm hoping someone has either found and fixed this problem before, or can steer me on how to diagnose it myself.

When editing a node, taxonomy autocomplete fields appear to hang - the spinner keeps going round forever. However, looking at the Network inspector, the lookup has returned "Connection Reset".

Further information:


Other kinds of autocomplete work on the site (e.g. node lookup when adding a redirect)
I am logged in as an admin user with full permissions to everything
The vocabulary is not empty
The autocomplete field is set to "create if term doesn't exist".
The form saves fine on submit, either creating a new term or working out which term to link to if I enter the exact text correctly
I am on the second-latest version (8.1.9)
Accessing the lookup URL directly from a separate tab gives "This site can't be reached / the connection was reset"
PHP error log - empty
Drupal "recent errors" log - empty
Behaviour same in Chrome and Firefox
Linux server, Windows client for browsers


The URL being accessed looks like this:
http://[mysite.co.uk]/entity_reference_autocomplete/taxonomy_term/default%3Ataxonomy_term/UhDfSC7PJL0Sf6NxfMCVY27CaUM15rrS-gVsL6eD66E?q=[search_term_here]

EDIT:

As requested, here's the list of contributed modules:


Protected Pages
AT Theme Generator
AT Tools
Admin Toolbar
Admin Toolbar Extra Tools
Adminimal Admin Toolbar
Chaos Tools
Chaos Tools Views
Chaos Tools Blocks
Bigpipe
Calendar
Calendar Datetime
Devel
Feeds
Audiofield
Computer field
Datetime
Image
Simple Google Maps
Image Effects
IMCE File Manager
Backup &amp; Migrate
Blog
Coffee
CookieConsent
Entity
Exclude Node Title
Libraries
Masquerade
PathAuto
Profile
Redirect
Redirect Import
Scheduler
Special Menu Items
Token
Twitter Block
Typed Data
User Display
Supercache
Rules
Security Review
Honeypot
Google Analytics
Flood Unblock
Superfish
Views PHP
Views Slideshow
Views Slideshow Cycle
Views templates
So after much frustration, I've finally got to the bottom of this. The answer is not within Drupal, but rather in my cPanel setup. I'll post the answer here because it may save someone else the headaches I've had.

The problem was that my cPanel hosting had xVarnish enabled. I'm not sure if it was suddenly added by a package upgrade, or if the behaviour changed, or if its effect only kicked in after a certain time span, but I certainly didn't switch it on! In any case, disabling xVarnish not only fixed this problem but also an "access denied" on image styles that had cropped up too.