This was an issue with 5.13 which has been reported and 5.13.1 has been released today to fix the issue.
I also am using 5.13.1 and am experiencing the 'No Constructor' error with the HTMLSnippet widget running locally. In the sandbox it works fine. Does somebody know what I need to do, to get this also running locally without the error?
For anyone who finds this thread and experiencing the ‘Constructor not found’ issue, it’s often the case that a widget is not perfectly working in Internet Explorer 11. Please check the JS console in IE to find out which widget is causing the problems. In my case it was the ClipboardTextCopy widget which was not compatible. There is a pull-request with a version that works, but it seems that the Github repo is abandoned… (Check for a working version: https://github.com/robertprice/ClipboardTextCopy)