Another update here for future reference. I had a look at the sources of codeblocks, it seems that error regex parsing is done per line. So to my opinion its not a quick fix to have multi-line regex processing as it takes quite some edits..
In the meantime I found a work-around, the IAR ARM compiler (iccarm) has an option --no_wrap_diagnostics that forces the error messages to be one line. The corresponding regex is then:
"(.*)",([0-9]+)[ \t]+Error\[(.*)\]:[ \t]*(.*)
Where the sub-expression indices are:
Message: 3
Additional message: 4
Filename: 1
Line: 2
Hope this helps others while configuring for IAR ARM