Not sure if it can really happen in a "normal" scenario but found the trace in the logs of a public instance.
Trace:
PHP Fatal error: Uncaught TypeError: Account_TimezonesCollection::isValidTimezone(): Argument #1 ($timezone) must be of type string, bool given, called in /usr/share/tuleap/src/common/User/Account/Register/RegisterFormHandler.php on line 78 and defined in /usr/share/tuleap/src/common/account/TimezonesCollection.class.php:27
Stack trace:
#0 /usr/share/tuleap/src/common/User/Account/Register/RegisterFormHandler.php(78): Account_TimezonesCollection->isValidTimezone()
#1 /usr/share/tuleap/src/common/User/Account/Register/RegisterFormProcessor.php(45): Tuleap\User\Account\Register\RegisterFormHandler->process()
#2 /usr/share/tuleap/src/common/User/Account/Register/ProcessRegisterFormController.php(66): Tuleap\User\Account\Register\RegisterFormProcessor->process()
#3 /usr/share/tuleap/src/common/Request/FrontRouter.php(242): Tuleap\User\Account\Register\ProcessRegisterFormController->process()