Hi,
You may not read Mapserver users list but this mail contains all relevant discussion so far
http://lists.osgeo.org/pipermail/mapserver-users/2014-August/076886.html
In short the issue is that if WMS layer name contain spaces like "my layer" then ArcGIS Server 10.2.2 accepts GetMaps only if the space is URL-encoded into "%20" but not if it is turned into "+". Mapserver developers consider that even according to http://www.ietf.org/rfc/rfc3986.txt both + and %20 are acceptable encodings in the query part of URL.
The change that seems to happen between ArcGIS 10.0 and 10.2.2 has broken cascading layers named like "my layer" with Mapserver. OpenJUMP is also using the + encoding and can't read those layers while QGIS can.
I made a quick test with Geoserver 2.6 RC1 and I suppose it has the same trouble. However, I could not capture from the Geoserver logs which kind of GetMaps it is sending for the remote WMS. Could some developer check which encoding we use, + or %20? If it is "+" is it something that should be changed? I am trying to contact ESRI about this.
I found also that for example letter "รค" can be URL-encoded into %E4 or into %C3%A4. ArcGIS Server accepts only the long version.
Here is a public ArcGIS WMS to test with
http://gtkdata.gtk.fi/arcgis/services/GTKWMS/MapServer/WMSServer?
-Jukka Rahkonen-