According to JFrog support,
In your request log we saw direst the api was called without specifying any repos. Note that indexing is resource intensive. Calculating and indexing for a repository may be a resource-intensive operation, especially for a large local repository or if the repository is a virtual one containing other underlying repositories.
Therefore, we recommend that you do not include repositories that do not require indexing for a periodic index calculation. Since you poke the indexing API every 15 minutes against all your repo. If your repo has increased in size to a point where each indexing requests took more than 15 mins, you may see the 500 error. Can you refine your choice of repo in your scheduled api call?
I don't see the unrestricted indexing requests they're referring to. There are requests like this one:
20171216011801|285|REQUEST|(IP address)|(user name)|POST|/api/maven|HTTP/1.1|200|0
The request is done by this command, explicitly specifying a single repository:
$ curl --fail -X POST -H 'Content-Length: 0' -u $USERNAME:$PASSWORD "https://repo.jenkins-ci.org/api/maven?repos=releases&force=1"
The Artifactory log confirms that it only indexes one actual local repo:
2017-12-16 01:18:01,321 [http-nio-8083-exec-248] [INFO ] (o.a.m.i.MavenIndexerServiceImpl:115) - Activating indexer for repo '[releases]' manually
2017-12-16 01:18:01,326 [art-exec-108213] [INFO ] (o.a.m.i.MavenIndexerServiceImpl:136) - Starting Maven indexing
2017-12-16 01:18:01,326 [art-exec-108213] [INFO ] (o.a.m.i.MavenIndexerServiceImpl:157) - Starting non virtual repositories indexing...
2017-12-16 01:18:01,327 [art-exec-108213] [INFO ] (o.a.m.i.MavenIndexerServiceImpl:159) - Non virtual repositories to index: [releases]
Notably, this restriction to a single local repo is a recent optimization attempt (from Wednesday/Thursday) I implemented in the hope of speeding up indexing. We used to request indexing on multiple explicitly specified repositories before Wednesday, and had done the same for several months when indexing was finished much quicker.
So, to recap: For multiple months, until Monday or Tuesday this week, indexing of multiple repos, was finished in ~15 minutes. Since Tuesday or Wednesday this week, indexing of a single local repo takes 1.5 hrs. Nothing from our side changed – in fact, we tried to speed it up some.