There is a Knowledgebase document referencing this error, and mentions antivirus issues specifically. However this could be more fundamentally related to a connection being dropped between the user and the server as well. Try to reindex the company file from the workstation experiencing the error. If issues persist, try deactivating any such antivirus software which may not be configured or the same as users not experiencing the issue, then reindexing. Also, a *.cdx file is the index portion of the *.dbf file in question, and reindexing via Sage can often help resolve errors related to index files. Below is a shortened link to the Sage Knowledgebase Article.
https://goo.gl/9ZJT8a