Where do you get the error? In the mailchimp interface you get a 500? or you are trying to call the url yourself?

This message is in mailchimp. It appears in red just below the webhooks callback url box after it fails its attempt to verify.

Notes:
I have it installed and working fine on a site previously and I can't see anything I've done different.
I do have admintools installed using their htaccess creator which sometimes blocks things but this is installed in the previous site also with no problems.
The only difference to the previous time I used CMC was that it was an older version.
Hope this helps

500 is an internal server error. So mailchimp is accessing the url, but for some reason the server crashes at this request.
I don't see anything fancy in the webhook script that could cause a 500 error. (like using a library or anything). Can you look at the error log com_cmc.webhooks.php? Do you see any message such as "wrong key" or something of that sort?

Maybe we are trying to fetch some data from the database and the database driver is throwing an exception.

Where do I find this log?

it shoulud be in your joomla folder and then in logs

That's where I checked. It doesn't exist there.

Go in your global config -> what's the path configured for log? Is the path existing, is it also writable? I remember that joomla was throwing a 500 error when it couldn't write to a log file. So maybe that is why the webhooks are crashing.

Awesome. This was indeed the error and all is working fine now.
We built a new site to replace the existing one and the log path was never updated after using is locally.
Thanks for your help.

Hi there,

I've got the same error at Mailchip webhook creation. I've checked "Path to Log Folder " at Joomla, it was set up for local host. So I've changed it by simply copying the url to the logs folder on my Control Panel. But the error at the MailChimp end remains. Currently I have 2 domains (one is for testing) and therefore there are two log files. Is that ok? or I should insert the file name as well to log path?
Thank you in advance.

Sorry, my error number is 502. The mrssage is "We couldn't verify the URL is working. Please double check and try again. HTTP Code: 502"
I've also checked the call back url at http://web-sniffer.net/, doesn't work there either ("Status: HTTP/1.1 502 Bad Gateway" - so the problem is not at MailChimp side. I even created a new API key to make sure.
Please help!
Thank you.

Problem solved! it was path to log issue.

Thanks for letting us know what it was Dina!