After upgrading my laptop to Kubuntu 10.10 I got another curious problem:
My nedit is not running in tabs but in separate windows per file. Whichever file I open first - I can copy/paste in this window.
In all other windows I cannot use copy and paste.
This bug is somewhat similar to #2784160 and several other kde/nedit bugs, but it's not the same.
ads@platin:~$ nedit -V
NEdit: Converting .nedit file to 5.6 version.
To keep, use Preferences -> Save Defaults
NEdit 5.6 CVS checkout release by Debian on Nov 18, 2008
Built on: Linux, Pentium Pro, GNU C
Built at: Nov 28 2010, 02:08:53
With Motif: (Untested) 2.1.0 [@(#)GNU/LessTif Version 2.1 Release 0.95.2]
Running Motif: 2.1 [unknown]
Server: The X.Org Foundation 10900000
Visual: 24-bit TrueColor (ID 0x21, Default)
Locale: de_DE.UTF-8
I believe lesstif's implementation of copy/pasting is buggy, the source code contains lots of "FIXME" strings in the region where copy and paste are implemented. Although your bug might be slightly different from other bugs, I still consider it due to the lesstif library.
Unfortunately until now nobody stepped up to help fix lesstif code (http://lesstif.sourceforge.net/#help).
I can confirm that the problem is gone after I installed OpenMotif and recompiled nedit against this lib.
It is usually a good exercise to try with OM (if possible) when any issue is encountered with Lesstif, especially the untested versions.