The widget is not for this mendix version. Upgrade to 5.10 or higher and the problem should be solved.
As far as I know the problem "only" exists on iOS devices. There also some improvements in version 5.14.
I've just tested and it does the same still (Galaxy Note4)
Do i have to download the widget again or is there a setting to change? I've tested this with 5.14 now.
I would like to confirm that I am experiencing the same problem (Modeler 5.15.1). The signature widget works perfectly in every browser I tryed and it works perfectly on iOS aswell. However I can't get it to work on Android. Not in a browser, nor in a published Phonegap application. A solution to this problem would be much appreciated.