There may be a certain interval between attempts, for example, 3 times every 8 hours. However, once the specified time has passed, the quiz may be available for you to take.
This does sound very strange. One theory would have to be that it’s some kind of bad state in your browser. You did mention that you tried clearing the cache and logging in again, so maybe it’s just a plain bug in your browser or in your browser plugins. I would suggest trying a completely different browser and different computer. E.g. if you’ve been using Chrome on a Windows laptop, try it using Safari on a Mac or Firefox on Windows or use a tablet instead of a computer.
The other theory is that the bad state is in your account on the Coursera website, so Tom’s suggestion of contacting the Coursera Help Center would be the next step if my other suggestion above doesn’t work.
We can “tag” some of the DLAI staff, but as Tom says it really doesn’t sound like this is anything that they would have any visibility into. It must be bad state on the Coursera website and I seriously doubt the course staff can do anything about.
Maybe the one hope of that strategy is that they would know whom to call directly at Coursera to get someone to look at this. I will send a direct message to a couple of people I know on staff and see if they can help.
Update: Seems like this is an identified issue and coursera development team in working to find a fix for it. This is the response I received.
"Upon reviewing our records, I am glad to share that this issue has already been reported to our engineering team, who is currently working to find a fix. Although I don’t have any further updates on the issue, I’ve added the details of your case to the existing report.
At the moment, there isn’t an estimated timeframe as to when this will be fixed. However, any update will be made automatically, so please check again in a few days to see if the changes have been made. In the meantime, you’re welcome to continue working through your courses."