We did have another customer report that they had a similar issue. Here's what he said to us.
I requested my host to disable mod_security which now allows me to update the plugin settings without the 404 error. I will have mod_security turned back on once the new settings are correctly tested, hopefully the plugin will continue to work.
We do not endorse the above suggestion, this customer suggested this was what he had tried and it worked for him, it may not work for you.
This customer was able to save all the settings with mod_security turned off, once he had finished testing the product and configured the extension appropriately he advised his host provider to turn it back on. You can read more about what mod_security is here.
In WordPress, after updating or saving a new post or existing post, sometime, it will redirect to a 404 page, and this is happening randomly, no idea to identify what is the root caused.
After read few similar posts – this and this, I find out this is due to the
mod_security filtering installed on Apache – If a post contains certain pre-defined dangerous “word combination” like “exec” or “SQL inject commands” like “insert”, the post will be filtered and WordPress just return a 404 page.
You can read a longer explanation on some tweaks you can make for your mod_security here.