Hi Han Pieter
The release notes from 5.16 https://world.mendix.com/display/ReleaseNotes/5.16.0
During the build custom widget are bundled. This step involves compiling the widgets against the official client API as documented here: https://apidocs.mendix.com/5/client/. Widgets that use undocumented functions of the mendix client will cause this step to fail and thus the project build to fail. You can use the logfile generated while bundling the widget resources to determine the cause of the problems. This logfile is available in your project in deployment/data/tmp/dojo/build-report.txt.
So check the build report and you will know which widget is troubling you (section errors)