Since when have you started experiencing this error. This might have happened after update of theme/any other plugin or compatibility issue with the theme/plugin/web server.
Firstly, I suggest you should try disabling all the plugins at once and then reactivating them one by one. This way you can find out which plugin is causing the issue.
Secondly, If deactivating/reactivating plugins does not solve the problem, then after that you can try switching the current theme of the website to a different one. You can also try disabling add-ons of the QSM, to check out whether they are causing the issue or not.
If above-mentioned resolutions do not work for you, you can try deleting and reinstalling the plugin. And you can export the plugin’s data to your computer before deleting it from the website and then import the data after reinstalling it, by using ‘Export & Import’ addon from our bundle.
But, if the issue is because of the web server, then you can try contacting the web server support team. Please do tell me whether these resolutions work out for you or not.