A couple of years later but maybe someone else can benefit from this.
I had the exact same problem @Taomach describes.
This was my error log:
[Wed Dec 30 20:16:48 2020] PHP Warning: Declaration of AddThisPlugin::register($category, $path) should be compatible with LazyLoadPlugin::register($category, $path, $mainContextId = NULL) in /Users/gabe/Code/local-evidencia-ojs-stable-3_1_2/plugins/generic/addThis/AddThisPlugin.inc.php on line 168
[Wed Dec 30 20:16:48 2020] PHP Warning: Declaration of CustomBlockPlugin::getBlockContext() should be compatible with BlockPlugin::getBlockContext($contextId = NULL) in /Users/gabe/Code/local-evidencia-ojs-stable-3_1_2/plugins/generic/customBlockManager/CustomBlockPlugin.inc.php on line 134
[Wed Dec 30 20:16:48 2020] PHP Warning: Declaration of RegistrationHandler::validate($request) should be compatible with PKPHandler::validate($requiredContexts = NULL, $request = NULL) in /Users/gabe/Code/local-evidencia-ojs-stable-3_1_2/lib/pkp/pages/user/RegistrationHandler.inc.php on line 19
[Wed Dec 30 20:16:48 2020] PHP Warning: Declaration of RegistrationForm::validate() should be compatible with Form::validate($callHooks = true) in /Users/gabe/Code/local-evidencia-ojs-stable-3_1_2/lib/pkp/classes/user/form/RegistrationForm.inc.php on line 21
[Wed Dec 30 20:16:48 2020] PHP Warning: call_user_func() expects parameter 1 to be a valid callback, class 'OldGreggThemePlugin' does not have a method 'htmlDisplay' in /Users/gabe/Code/local-evidencia-ojs-stable-3_1_2/lib/pkp/classes/plugins/HookRegistry.inc.php on line 107
[Wed Dec 30 20:16:48 2020] PHP Notice: Undefined property: Smarty_Internal_Undefined::$objMap in /Users/gabe/Code/local-evidencia-ojs-stable-3_1_2/lib/pkp/lib/vendor/smarty/smarty/libs/sysplugins/smarty_internal_extension_handler.php on line 132
[Wed Dec 30 20:16:48 2020] PHP Fatal error: Uncaught --> Smarty: undefined extension class 'Smarty_Internal_Method_Register_Outputfilter' <--
thrown in /Users/gabe/Code/local-evidencia-ojs-stable-3_1_2/lib/pkp/lib/vendor/smarty/smarty/libs/sysplugins/smarty_internal_undefined.php on line 62
[Wed Dec 30 20:16:48 2020] [::1]:56236 [500]: /index.php/Evidencia/user/register - Uncaught --> Smarty: undefined extension class 'Smarty_Internal_Method_Register_Outputfilter' <--
thrown in /Users/gabe/Code/local-evidencia-ojs-stable-3_1_2/lib/pkp/lib/vendor/smarty/smarty/libs/sysplugins/smarty_internal_undefined.php on line 62
This post enlightened me about a conflict with the ORCID plugin. Disabling or updating ORCID plugin to my ojs version (3.1.2) corrected the situation.