Completed 500 Internal Server Error

[kha0s178]

On some projects i've got subj after click on "Test Cases" tab. In logs it looks like:

[Redcase] Time calculation: Convert root test suite to json

Completed 500 Internal Server Error in 769.3ms

On other project it works and in logs i see:

[Redcase] Time calculation: Convert root execution suite to json

0.0038 sec(s): Convert root execution suite to json

What could be wrong? How could i fix it?
Thanx!