Intuit have now fixed this.

I sent them a message through their Facebook page and the guys there got me to go through the whole browser cache clearing routine again (which is really frustrating when you know that's not the problem). However they then got me to delete the link and even after this they told me that it was still showing as connected at their end, so they raised it to their technical guys. I don't know what they did but it is now fixed (took 3 or 4 days).

Like you I wondered whether it was to do with the GDPR changes, but the link to Barclaycard has always been a bit flakey - not like those to my HSBC accounts which I never seem to have a problem with.

So if you are still havign problems I suggest sending them a message via Facebook.