Hi Robbe,
Sorry for the inconvenience caused.
I was not able to reproduce the issue here.I used the following steps to reproduce the issue in the attached sample.
Step 1: Run the attached sample
Step 2:Load the form contains the usercontrol(in docked state) by menu.
Step 3: Add the Panel by menu.
Step 4:Try to dock the panel to the form contains the userControl.
Please follow the above steps and let me know, if i understood you correctly.
DockingMgr_UserControl.zip
Thanks for using Syncfusion products.
Regards,
Malarvizhi