Hello Matthias,
we're also investigating in the new Rich Text Editor "TinyMCE" in SAP CRM and have similar problems.
Is following ICF servive active at our system?
/sap/public/bc/ui5_ui5 [-> Found at CL_THTMLBX_BTF_EDITOR=>DELEGATE_EDITOR()]
If yes, what expierence did you made with the new editor? 
Regards
Dieter