Jan 28 2016
Llyrian confirmed that he can do it tommorow.
Jan 27 2016
We solved this issue. The problem was the name of the data type. When the Chrome bug appeared, the name changed from 'PT:math' to 'Math (TeX)' (and finally to 'Mathematical Expression'). The property P59844 used in our test item Q117940 'math testing' was created when the name of property's data type was 'PT:math'. It looks like Chrome was cashing it into the localstore as Thiemo said. So, everybody who used the property in chrome couldn't edit nor add it after the data type's name changed to 'Math (TeX)'. Furthermore it seems like it was a special problem wit the prefix 'PT:'. But it's just a side note.
Jan 26 2016
I can reproduce the error 'It is not possible to define a new value for a deleted property.' in vagrant (http://wikidata-math-de.wmflabs.org/wiki/Q15) when I try to add an old property (P4) of property type 'PT:math', which is obsolete, because the name has changed to 'math'. This happened in chrome and Firefox on Win 7. So I created a new property of property type 'math' (P10). I can add and edit it to any item. So it work's again in vagrant.
The 'propblem' with IE 11 on Win 7 seems to be another issue. In IE 11 I can't edit or add any property of any item. So: editing / adding in wikidata on wmflabs doesn't work at all for me with IE 11 on Win 7.
Jan 22 2016
Tested on Windows 7 with