Thanks for the replies. We've now received confirmation from OpenText
that it's a bug and due to be fixed in August.
If anyone else has this issue, the problem is that in CMS 10 there are
2 display handlers that run in different directories: /cms and /cms/
WebClient.
We were linking to our CSS like this:
<link href="<%con_LayoutCSS%>" rel="stylesheet" type="text/css" />
which produced code like this:
<link
href="RDTemp_4AA9CBD10E7345FE95B24C6FBA1AF9F2_0BFFC5971CCB4B18883D4D3877C540FF.css"
rel="stylesheet" type="text/css" />
So, because the CSS link is relative to the current directory, but the
display handlers are in 2 different directories it's not going to work
in both.