I have my personal fork of a project, and there is the 'main' repo, both of which are on bitbucket. I'm currently getting a 500 error when cloning one of them, and then pulling from the other. I have not seen this issue with this repository before - I have been happily pulling and pushing from multiple forks, and I don't believe anything has changed about my setup, though obviously the repositories have continued to change over time. Here is the example. I know there is not much detail, but you may be able to reproduce it with these two repositories (they are public).
[bilbo:~]$ cd /tmp
[bilbo:tmp]$ hg clone https://bitbucket.org/labscript_suite/labscript_utils
destination directory: labscript_utils
requesting all changes
adding file changes
added 701 changesets with 744 changes to 96 files
new changesets 81b41521ad5e:783680f0d6f0
updating to branch default
58 files updated, 0 files merged, 0 files removed, 0 files unresolved
[bilbo:tmp]$ cd labscript_utils/
[bilbo:labscript_utils]$ hg pull https://bitbucket.org/cbillington/labscript_utils
pulling from https://bitbucket.org/cbillington/labscript_utils
searching for changes
abort: HTTP Error 500: Internal Server Error
[bilbo:labscript_utils]$ hg --version
Mercurial Distributed SCM (version 5.0)
(see https://mercurial-scm.org for more information)
Copyright (C) 2005-2019 Matt Mackall and others
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
I am seeing this problem with mercurial 5.0 on Linux, but not with mercurial 4.9 on Windows. My Linux computer has been on mercurial 5.0 for 3 weeks, and I have been pulling and pushing from both these repos in that time.