Trying installing the ISRG Root X1 certificate on a test machine to see if that helps:
- browse to http://x1.i.lencr.org/ in order to download the .cer file for ISRG Root X1 (your browser may warn about the file type and you may need to click "Keep" to save the file)
- open file, click "Install Certificate..", Choose default option "automatically select..", Next, Finish
- Reboot
- Test browser again.
If this works, you need to ensure that all of the machines you manage have this certificate installed.