I have the same problem exactly. I just downloaded Mendix 5.15.0 and the company expenses project from appstore. It was working fine for 1 or 2 days and then this issue happen.
Did you create a new theme for your Mx5 model? Did you take all the steps in the documentation about moving from 4 to 5? ( https://world.mendix.com/display/refguide5/Moving+from+4+to+5)
I've cleaned up some widgets and still have the issue, I did start seeing this error:
404 - file not found for file: mxclientsystem/templatesystem/widget/ImageViewer.js
Was able to narrow my issue down to the formtooltip widget. Once I removed, I was able to log in.