Thanks, Acorn! It turns out that my problem was caused by a browser extension called "I still don't care about cookies":
https://addons.mozilla.org/en-US/fir...eaboutcookies/
https://chromewebstore.google.com/de...dfdjjppcpchdlm

I found that the problem is solved by adding helpandsupport.xara.com to the whitelist in the options for this extension, or by disabling the extension. I trusted this extension because it hadn't caused any trouble previously, but now I shall be quicker to suspect it.