submit functionality stopped working

The contribution list web parts on our site have worked fine for several months. However it appears now it's broken on submit of the form. The same error is occurring with the contribution list web part on multiple pages, so it's not caused by changes specific to a single page. I have been looking at various settings and can no identify a potential root cause.

It also appears that any web part with a submit functionality also no longer submits. The submit btn does nothing on click(no console errors either). On the server side, no Kentico event logs are created for them.

Is there a global setting that can potentially affect all of these functionalities?

Error log below was generated on clicking "Add" of the contribution list form:
Note - When submitting the form, all fields were populated despite what the message says.

EventType: E
Source: Content
EventCode: SaveDocument
EventDescription: An error occurred when saving data. Please see event log for more details.
Message: Value cannot be null.
Parameter name: input
Exception type: System.ArgumentNullException

Stack Trace: 
   at System.Text.RegularExpressions.Regex.Replace(String input, String replacement)
   at CMSFormControls_Basic_HtmlAreaControl.get_ValueForValidation()
   at CMS.FormControls.FormControlValidation.LoadControlValue()
   at CMS.FormControls.FormControlValidation.Validate()
   at CMS.FormControls.BasicForm.ValidateData()
   at CMS.FormControls.CMSForm.DocumentManager_OnValidateData(Object sender, DocumentManagerEventArgs e)

at System.EventHandler 1.Invoke(Object sender, TEventArgs e) at CMS.FormControls.CMSAbstractManager`2.RaiseValidateData(EventArgsType args) at CMS.FormControls.CMSDocumentManager.SaveDocumentInternal(String actionContext)