Please do the following to find out the cause.

1- Ensure both units are at the same firmware level -  v.1.5.146 or above

2- Use same model of KVM cables for both units [2L-520xP or 2L-520xU]

3- Check or replace the cascading cable with a known good one.

4- Break the cascade and verify each unit works independently. 

5- Swap each other to find out if one of them is causing the issue.