Blank Dialog Box??

I downloaded Ultimate Maintenance Mode while I set up my shop. When I turned it off, I noticed that I would get this random blank dialog/conversation box that would say “The page at serumno5 says” then it’s blank with the OK button. I have never seen this before downloading this plugin. I tried to delete the plugin, but that box is still there and it really slows down the load time of the site. Has anyone else ever had this problem? How do I fix it? To see what I’m talking about the site is https://www.serumno5.com

https://www.ads-software.com/extend/plugins/ultimate-maintenance-mode/