I am using Joomla 3.9.20 and BF 1.9.1 Stable (build 939). After installing the last update, we are getting 406 errors when attempting to add a zapier webhook. This is something that we have done regularly for some time. Also, existing forms seem to be functioning correctly.
yes, this seems to be Zapier unrelated, because at this point (saving more options), there is no communication going on with Zapier.
So it must be something server specific.
Can you please find your server's error logs? Especially the PHP error logs.
There is also a joomla error log in the logs folder called "error.php" that I would need to rule out if joomla itself is doing this.
Last but not least, do you probably have a security extension like Admin Tools installed?