This is a super, duper important troubleshooting step. Every tech support person will start their conversation with you by asking if you’ve tried deactivating plugins and for good reason. Most problems are a result of plugins not doing what they’re supposed to or not being developed with considerations for unusual circumstances.
Complete this step by navigating to the Plugins list page in your dashboard (usually yoursite.com/wp-admin/plugins.php) and deactivating plugins one by one. Start with the plugins you consider to be most likely, like ones alluded to in error messages or ones which introduce functionality relating to the problem area. After each plugin is deactivated, test again to see if the problem is gone.