This error could occur when the Denodo service was not started as an administrator or does not have privileges over the <DENODO_HOME> Directory. When you restarted the services (probably as an administrator) the problem looks to be resolved.
Next time when this error occurs, you could check the vdp.log file present in the path <DENODO_HOME>/logs/vdp/ for troubleshooting the error message.
To configure the log you could refer to the documentation [Configuring the Logging System](https://community.denodo.com/docs/html/browse/7.0/vdp/administration/appendix/configuring_the_logging_engine/configuring_the_logging_engine) and KB article [How to log the request issued on a VDP Server](https://community.denodo.com/kb/view/document/How%20to%20log%20the%20requests%20issued%20on%20a%20VDP%20server?category=Operation).
Hope this helps!