InfoPath Access Denied form templates (SharePoint, File server, Local)

Question

We have multiple InfoPath forms which have recently started to not open with Access Denied messages commonly being the result. This is at the template side, vs the form side, but the effect is the same. In testing, we have shifted templates to local disk in case this was a SharePoint problem, but the behavior is unchanged. Error message captured in Office Alerts are:
Event ID 300:

InfoPath Editor

InfoPath cannot open a new form.

The form cannot be opened. To fix this problem, contact the form designer.

P1: 1201180

P2: 15.0.4805.1000

P3:

P4:

There is no clear reason for the error, and I've not found a way to work around this. I have in a few cases been able to create a working template that allows me to get around the error, but that seems to work less and less. This was using local files. InfoPath version is latest update available; SharePoint is 2013, we're actively working to get out of this, but to complete that, we're working with InfoPath to extract our data (print to PDF), which these errors make really challenging. IE security settings are set to Trusted sites for the SharePoint locations, and we've included configurations for Intranet settings as well. Doesn't seem to have any affect.

Steps done so far:

1. copied template to local computer, tested template. Access Denied.
2. Updated template and published a change to a local path, Access denied
3. Published to a shared network location, Access denied.
4. Review the content in the template, strip ALL automation, external data reference and SharePoint elements from the template. Save and publish (local and Network Share), Access denied.
5. Set Form security to full trust, signed using a trusted certificate, save, publish. Access denied.

Hoping the community might have ideas we've not tried.