Previously, some binary Python modules used by Horizon were compiled with a flag that requires them to use the simplified GIL state API. Since mod_wsgi uses sub-interpreters and the simplified GIL state API is not reentrant, it results in the mod_wsgi locking up. Consequently the dashboard is unavailable.
With this update, the mod_wsgi configuration was changed, forcing mod_wsgi to use the main interpreter instead of sub-interpreters. As a result, mod_wsgi works correctly.