Error: empty return for method 'ExecuteQuery' xtk:queryDef

We are frequenctly getting a lot of these errors:

1. Error: empty return for method 'ExecuteQuery' xtk:queryDef

2. Enpyt return for method ‘GetEntityIfMoreRecent’ of service 'xtk:persist'

We checked on the community for similar errors and found this:
Solved: Error: empty return for method 'ExecuteQuery' xtk:... - Adobe Experience League Community - 300494

But we have the same client and server and everything mentioned in the above post is checked out. 

1.

2.

Due to these errors we are also getting errors in jssp pages:

1__________________________________________________ INT-150012 The HTTP query returned a 'Forbidden' type error (403) <JSfunction> - Error while select <schema> record :0 ___________________________________________________

2___________________________________________________ <JS function> - Error while update <schema> record :119210626 - 119459925 - SOP-330012 Empty return for method 'Write' of service 'urn:xtk:session'  _____________________________________________________

3____________________________________________________  :SOP-330012 Empty return for method 'ExecuteQuery' of service 'urn:xtk:queryDef' _____________________________________________________

4____________________________________________________<JS function> - Error while inserting <schema> record :358465841182 - EI - 119210626 - SOP-330012 Empty return for method 'Write' of service 'urn:xtk:session' 



Any idea as to why we are getting these???? And what can be done to resolve this?