Few versions of apt-cacher-ng did loose some information about the original source of downloaded files. This has been unnoticed for many months, because the path resolution algorithm tries to use a file path components go guess the original URL, or guess the cache repository and then download from using this repository (backends definitions).
The good news is that the missing information is completed automatically when a file is redownloaded once (or resumed) through apt-cacher-ng. Static package files are usually not redownloaded, but the missing information is not relevant for them.
The bad news is that in some cases this might become a problem, particularly when an expiration/import tasks tries to refresh the index file which is stored inside of a (former) repository tree and the administrator has removed the backends definitions for this repository. When ACNG runs out of possible sources it reports "Code 520824" and refers to this text. However, the detection of this situation is still not perfect and in rare cases it's possible that messages about failed resolution of some hostname (identical with former cache repository name) will appear.