Hi,
Thank you for reopen old thread, it shows that you've made search before posting questions.
I suppose, if you test with ignore enhancement user parameter, the issue won't be there, am I right?
If my guess is correct, please try to check if you have enhanced this assignment block. Please test to ignore the enhancement for this assignment block only. You may check in How to check and identify whether an issue on CRM WebUI is caused by enhancement of some component or view - CRM - SCN W… regarding how to test by deactivating enhancement for a view only.
I've ever encountered one issue, that is customer coding in context node leads to an exception, which leads to the issue.
Hongyan