Seems like mistake on user end.

If it was confirmed, the way to prevent this would be to: run our automated piwik.js tests on more browsers than just webkit (modern IE, firefox, ...) #9874

@mattab

Issue is reproducible on http://m4g.gbros.pl/ec/site1a using IE 10.0.9200.17556 and 11.212.10586.0

This problem occurs around here:
https://github.com/piwik/piwik/blob/master/js/piwik.js#L3115

For isSitePath(path, aliasPath) there is a situation when
path = '/site1/path/(...)'
and
aliasPath = 'site1/path/(...)'

There is a '/' missing. I hotfixed that on my test instance by something like that:
if (stringStartsWith(path, '/') && !stringStartsWith(aliasPath, '/')) { aliasPath = '/' + aliasPath; }
just before this https://github.com/piwik/piwik/blob/master/js/piwik.js#L3115 condition. Function stringStartsWith was of course introduced somewhere above.
I will create a PR for reviews tomorrow.