Though OPA 10.4 features are more or less same, there is a difference in compilation between OPA 10.4.0 rules and OPA 10.4.2 rules.
In 10.4.2 we can write an entity rule -
Exists (<relationship>, <condition>)
in 10.4.0, the same rule would give an error. There should be no space between Exists and "(" in 10.4.0.
This leads to the error - "No entity with name '' found in <rulebase>".
These errors happen when 10.4.2 rules are opened in 10.4. The only sane solution is to upgrade the OPA version.
There are no structural / implementation changes between these versions, just that few such issues are resolved in higher versions of 10.4.
