I've searched the forum (and the rest of the web as well) and I've found a related topic.
https://community.magento.com/t5/Magento-2-x-Admin-Configuration/Can-t-access-or-manage-categories-a...
My issue is similar. Here is the report from /var/report/ to this to see if anyone can figure what's wrong.
{"0":"Class TemplateMonster\\Megamenu\\Model\\Attribute\\Backend\\Configurator does not exist","1":"<pre>#1 Magento\\Framework\\Code\\Reader\\ClassReader->getConstructor() called at [vendor\/magento\/framework\/ObjectManager\/Definition\/Runtime.php:49]\n#2 Magento\\Framework\\ObjectManager\\Definition\\Runtime->getParameters() called at [vendor\/magento\/framework\/ObjectManager\/Factory\/Dynamic\/Developer.php:48]\n#3 Magento\\Framework\\ObjectManager\\Factory\\Dynamic\\Developer->create() called at [vendor\/magento\/framework\/ObjectManager\/ObjectManager.php:56]\n#4 Magento\\Framework\\ObjectManager\\ObjectManager->create() called at [vendor\/magento\/framework\/Validator\/UniversalFactory.php:36]\n#5 Magento\\Framework\\Validator\\UniversalFactory->create() called at [vendor\/magento\/module-eav\/Model\/Entity\/Attribute\/AbstractAttribute.php:596]\n#6 Magento\\Eav\\Model\\Entity\\Attribute\\AbstractAttribute->getBackend() called at [generated\/code\/Magento\/Catalog\/Model\/ResourceModel\/Eav\/Attribute\/Interceptor.php:1155]\n#7 Magento\\Catalog\\Model\\ResourceModel\\Eav\\Attribute\\Interceptor->getBackend() called at [vendor\/magento\/module-eav\/Model\/ResourceModel\/ReadHandler.php:135]\n#8 Magento\\Eav\\Model\\ResourceModel\\ReadHandler->execute() called at [vendor\/magento\/framework\/EntityManager\/Operation\/Read\/ReadAttributes.php:59]\n#9 Magento\\Framework\\EntityManager\\Operation\\Read\\ReadAttributes->execute() called at [vendor\/magento\/framework\/EntityManager\/Operation\/Read.php:111]\n#10 Magento\\Framework\\EntityManager\\Operation\\Read->execute() called at [vendor\/magento\/framework\/EntityManager\/EntityManager.php:70]\n#11 Magento\\Framework\\EntityManager\\EntityManager->load() called at [vendor\/magento\/module-catalog\/Model\/ResourceModel\/Category.php:1077]\n#12 Magento\\Catalog\\Model\\ResourceModel\\Category->load() called at [generated\/code\/Magento\/Catalog\/Model\/ResourceModel\/Category\/Interceptor.php:336]\n#13 Magento\\Catalog\\Model\\ResourceModel\\Category\\Interceptor->load() called at [vendor\/magento\/framework\/Model\/AbstractModel.php:540]\n#14 Magento\\Framework\\Model\\AbstractModel->load() called at [generated\/code\/Magento\/Catalog\/Model\/Category\/Interceptor.php:1480]\n#15 Magento\\Catalog\\Model\\Category\\Interceptor->load() called at [vendor\/magento\/module-catalog\/Controller\/Adminhtml\/Category.php:56]\n#16 Magento\\Catalog\\Controller\\Adminhtml\\Category->_initCategory() called at [vendor\/magento\/module-catalog\/Controller\/Adminhtml\/Category\/Edit.php:77]\n#17 Magento\\Catalog\\Controller\\Adminhtml\\Category\\Edit->execute() called at [generated\/code\/Magento\/Catalog\/Controller\/Adminhtml\/Category\/Edit\/Interceptor.php:24]\n#18 Magento\\Catalog\\Controller\\Adminhtml\\Category\\Edit\\Interceptor->execute() called at [vendor\/magento\/framework\/App\/Action\/Action.php:108]\n#19 Magento\\Framework\\App\\Action\\Action->dispatch() called at [vendor\/magento\/module-backend\/App\/AbstractAction.php:231]\n#20 Magento\\Backend\\App\\AbstractAction->dispatch() called at [vendor\/magento\/framework\/Interception\/Interceptor.php:58]\n#21 Magento\\Catalog\\Controller\\Adminhtml\\Category\\Edit\\Interceptor->___callParent() called at [vendor\/magento\/framework\/Interception\/Interceptor.php:138]\n#22 Magento\\Catalog\\Controller\\Adminhtml\\Category\\Edit\\Interceptor->Magento\\Framework\\Interception\\{closure}() called at [vendor\/magento\/module-backend\/App\/Action\/Plugin\/Authentication.php:143]\n#23 Magento\\Backend\\App\\Action\\Plugin\\Authentication->aroundDispatch() called at [vendor\/magento\/framework\/Interception\/Interceptor.php:135]\n#24 Magento\\Catalog\\Controller\\Adminhtml\\Category\\Edit\\Interceptor->Magento\\Framework\\Interception\\{closure}() called at [vendor\/magento\/framework\/Interception\/Interceptor.php:153]\n#25 Magento\\Catalog\\Controller\\Adminhtml\\Category\\Edit\\Interceptor->___callPlugins() called at [generated\/code\/Magento\/Catalog\/Controller\/Adminhtml\/Category\/Edit\/Interceptor.php:39]\n#26 Magento\\Catalog\\Controller\\Adminhtml\\Category\\Edit\\Interceptor->dispatch() called at [vendor\/magento\/framework\/App\/FrontController.php:159]\n#27 Magento\\Framework\\App\\FrontController->processRequest() called at [vendor\/magento\/framework\/App\/FrontController.php:98]\n#28 Magento\\Framework\\App\\FrontController->dispatch() called at [vendor\/magento\/framework\/Interception\/Interceptor.php:58]\n#29 Magento\\Framework\\App\\FrontController\\Interceptor->___callParent() called at [vendor\/magento\/framework\/Interception\/Interceptor.php:138]\n#30 Magento\\Framework\\App\\FrontController\\Interceptor->Magento\\Framework\\Interception\\{closure}() called at [vendor\/magento\/framework\/Interception\/Interceptor.php:153]\n#31 Magento\\Framework\\App\\FrontController\\Interceptor->___callPlugins() called at [generated\/code\/Magento\/Framework\/App\/FrontController\/Interceptor.php:26]\n#32 Magento\\Framework\\App\\FrontController\\Interceptor->dispatch() called at [vendor\/magento\/framework\/App\/Http.php:137]\n#33 Magento\\Framework\\App\\Http->launch() called at [generated\/code\/Magento\/Framework\/App\/Http\/Interceptor.php:24]\n#34 Magento\\Framework\\App\\Http\\Interceptor->launch() called at [vendor\/magento\/framework\/App\/Bootstrap.php:261]\n#35 Magento\\Framework\\App\\Bootstrap->run() called at [pub\/index.php:40]\n<\/pre>","url":"\/admin_1sxmen\/catalog\/category\/index\/key\/6c974f36a9fb7e99761bcb0d2d91044e4a220bd467d4d880b6139b15fc5ab818\/","script_name":"\/index.php"}I found and deleted the related row from eav_attribute table, but it didn't help to me.
