This is because oXygen 18.0 was updated to support version 2.2.3 DITA Open Toolkit and the default DITA setting was changed to use version 2.2.3. If your DITA documents rely on an XML catalog that references an older version of DITA Open Toolkit, then Acrolinx will not be able to check your documents.
You can use either of the following workarounds to manage this oXygen limitation:
Use Author mode when checking and everything should work fine.
On the oXygen Preferences > DITA page, change the default DITA setting from 2.x to 1.8. oXygen will then use an older version of DITA Open Toolkit to resolve your document's references and Acrolinx will be able to analyze your documents.