You can execute the query again with trace on and check 'View execution trace' or the trace in VQL shell.
For example, in the VQL shell, appending the word trace to the query will enable the trace,
SELECT * FROM <view_name> trace;
You can refer to '4.6.1 Execution Trace of a Statement' in the 'Virtual DataPort Administration Guide.pdf' (Can be downloaded from https://community.denodo.com/docs/view/document/Virtual%20DataPort/Denodo%205.5/Virtual%20DataPort%20Administration%20Guide)
By using the trace it will be easier to see what caused the error.
Hope this helps.