.map files are being read by javascript debugger engines (I’ve seem them with Chrome and Edge (not sure about Firefox). For Chrome this happens usually after opening the developer window.
.map files are a de-facto standard to rebuild source code from minimized javascript files. As there’s no source code structure for mxui.js (at least non for the non-Mendix guys), those errors ca be simply ignored...
So if you see those messages, don’t worry.