Release Automation 6.7 or higher
The application logs are limited in term of how much data they can hold at directory level and as well as at file level. Once that limit is reached, the purge/truncation of the application logs will done by system.
In case if you try to download the data purged it will show the mentioned error. The file size limit can also result in some overwriting or truncation.
These properties are configurable, please refer to section Execution Log Control in the documentation: