I've been getting this error whenever a repo has a commit, I've tested this out with git and subversion repos both new and old.
Started happened after a full update.
test command: <phabricator>/scripts/repository/reparse.php --message --change --owners --herald --force rCHC43c0f87cad5120fc4de88ecab20c122e9a577ad5 --trace
NOTE: repo codename: CHC; revision: 43c0f87cad5120fc4de88ecab20c122e9a577ad5; repo type: Git
[2014-08-11 01:55:42] EXCEPTION: (RuntimeException) Object of class DifferentialParseCommitMessageConduitAPIMethod could not be converted to string at [<phutil>/src/error/PhutilErrorHandler.php:205] #0 PhutilErrorHandler::handleError(integer, string, string, integer, array) #1 vsprintf(string, array) called at [<phutil>/src/internationalization/PhutilTranslator.php:91] #2 PhutilTranslator::translate(string, DifferentialParseCommitMessageConduitAPIMethod, string) #3 call_user_func_array(array, array) called at [<phutil>/src/internationalization/pht.php:17] #4 pht(string, DifferentialParseCommitMessageConduitAPIMethod, string) called at [<phabricator>/src/applications/conduit/protocol/exception/ConduitApplicationNotInstalledException.php:11] #5 ConduitApplicationNotInstalledException::__construct(DifferentialParseCommitMessageConduitAPIMethod, string) called at [<phabricator>/src/applications/conduit/call/ConduitCall.php:181] #6 ConduitCall::buildMethodHandler(string) called at [<phabricator>/src/applications/conduit/call/ConduitCall.php:21] #7 ConduitCall::__construct(string, array) called at [<phabricator>/src/applications/diffusion/query/lowlevel/DiffusionLowLevelCommitFieldsQuery.php:23] #8 DiffusionLowLevelCommitFieldsQuery::executeQuery() called at [<phabricator>/src/applications/diffusion/query/lowlevel/DiffusionLowLevelQuery.php:23] #9 DiffusionLowLevelQuery::execute() called at [<phabricator>/src/applications/repository/worker/commitmessageparser/PhabricatorRepositoryCommitMessageParserWorker.php:49] #10 PhabricatorRepositoryCommitMessageParserWorker::updateCommitData(DiffusionCommitRef) called at [<phabricator>/src/applications/repository/worker/commitmessageparser/PhabricatorRepositoryGitCommitMessageParserWorker.php:15] #11 PhabricatorRepositoryGitCommitMessageParserWorker::parseCommit(PhabricatorRepository, PhabricatorRepositoryCommit) called at [<phabricator>/src/applications/repository/worker/PhabricatorRepositoryCommitParserWorker.php:44] #12 PhabricatorRepositoryCommitParserWorker::doWork() called at [<phabricator>/src/infrastructure/daemon/workers/PhabricatorWorker.php:87] #13 PhabricatorWorker::executeTask() called at [<phabricator>/scripts/repository/reparse.php:282]