This error is caused because 3dm2 uses a no longer supported SSL cipher. Normally in order to fix this issue you would just issue an updated SSL cert but 3DM2 does not offer this option, so you will need to open up a legacy browser.
On windows you will need to run internet explorer
then open the local server address, https://localhost:888/
It is possible you will need to go into internet explorer > internet options > advanced and enable addition SSL protocols as well.