Floor van Gageldonk
There must be something wrong with the deployment location of your.
First try to clear your cache and refresh the form.
If that doesn't work try to remove the file modelerversion.txt file, re-deploy the project, empty your cache, refresh and see if you can login now.
Do you use a theme?? Then the problem might be in the theme. Deploy the project without the theme once. If you can login without the them then you know where the problem is.
Now the solution if the theme is your problem. There have been some changes in the index.html files.
Compare the files index.html and index3.html from your theme with the files deployed by the modeler.
The index files deployed by the modeler contain some additional lines like this: