Ab,
the problem is an error in the FTB FreeTextBox Controls.
It seems that DNN 3.0.12 and 3.0.13 did not include the latest version of FTB control.
Please have a look at your BIN folder and check the version of your freetextbox.dll.
If the version of the dll is 3.0.5000.1 it is not the last version.
The last version (in which the problem you describe is solved) ist the version 3.0.5000.3.
I have recompiled an actual version of the Ftb3HtmlProvider for DNN 3.X and have now include the actual version of freetextbox.dll.
To solve your problem download the Advanced FTB Provider Update for DNN 3.0.13 (now include the new ftb dll 3.0.5000.3) from my web site and copy the content with all subdirectories to the bin folder of your dnn installation.