Hi Sascha,
Thank you for posting your query to us.
I am afraid that I was not able to reproduce the issue. Please refer to the following steps that I tried to reproduce it.
1. Created a sample with callback panel and genericdropdown
2. Then added a treeviewcontrol into this genericdropdown
3. Added some nodes in the callback-panel-refresh-procedure
4. Run the sample
5. No issues.
Also I have created a simple sample to test this issue and it is available in the following link.
http://websamples.syncfusion.com/samples/Tools.Web/5.1.1.0/66333_1/main.htmPlease have a look at the above sample and if still the issue exists, could you please try reproducing it in the above sample or send us the reproducing steps so that we could sort out the cause of the issue and provide you a solution?
Thanks,
Melba