For Air-Gapped deployments, there appears to be a bug when using the source code matching within the analysis workbench. You can replicate the issue by enabling the source code matching for a given file then select the 'Download Remote File' button. Without any internet, it pops up the HTTP 500 error code. Note that the 'Compare with Remote File' actually handles the exception with stating the cannot access remote file so a similar solution should be made for the 'Download Remote File' action as well.
Tested with FNCI 2018.R4
Thanks for calling this out. This issue can be tracked as SCA-3432 and will be prioritized into an upcoming release.
I'm glad to see someone is taking advantage of running Code Insight in an air-gapped environment since it's one of the few solutions that supports running in 'offline' mode without a significant loss in functionality!