See PHI1066. See <https://discourse.phabricator-community.org/t/maniphest-is-giving-http-500-error/2385>. See D20116. Some datasources currently rely on `Datasource->setParameters(...)` accepting nonscalar values (usually, lists of PHIDs). This no longer works after D20049 and fatals explicitly after D20116.
Affected typeaheads include:
- The branch selector in "Land Revision".
- Some custom field search typeaheads, e.g. on `/maniphest/` with particular custom fields configured.
The exception itself is also opaque:
```
>>> UNRECOVERABLE FATAL ERROR <<<
Method PhutilURI::__toString() must not throw an exception
/home/.../phabricator/src/applications/typeahead/datasource/PhabricatorTypeaheadDatasource.php:0
┻━┻ ︵ ¯\_(ツ)_/¯ ︵ ┻━┻
```