Recently, I ran into some trouble with my WordPress installation that resulted in an error branded “Check your redirect URI setting”. This error occurs whenever I try to edit the settings within the OptinMonster plugin or when I try to authorize Jetpack with my WordPress.com account. The Jetpack authorization issue wasn't a huge deal as I only really needed to do it once, but it became a large issue when I couldn't make any changes to my OptinMonster settings.
After a few hours of web research, I had trouble finding any helpful information on the issue although it did appear that many others were experiencing the same issue. Soon enough, I discovered the issue had to do with a plugin conflict so I enabled/disabled each of my active plugins until I discovered the culprit.
In my case, it was a plugin called Intense Site Builder of which I purchased from Code Canyon a few months back. Since this plugin was produced by a sole developer rather than an established company, it was more prone to cause plugin conflicts. If you're in a similar situation, I'd recommend activating and deactivating your plugins starting with the least popular ones. Not all situations will be alike and many different plugins have the potential to cause this issue on a WordPress installation so each individual case will vary in how to go about stopping it.
Now that I have pinpointed which plugin causes the issue, I simply disable it when I need to make any changes to OptinMonster and then re-enable it when I am finished with the changes. While I lose the functionality of the Intense Site Builder plugin temporarily, I am still able to utilize both plugins simultaneously for the majority of time and was not forced to drop the Intense Site Builder plugin altogether.
I hope this helps any of you who may be stuck with this WordPress error! If so, I'd appreciate you leaving a comment as to which plugin (or other non-plugin related issue) was responsible for this particular error on your website. This way you'll get some good karma for helping others speed up their troubleshooting process.