Cannot connect via listener but can tnsping listener

I'm trying to connect to a remote machine using tnsnames but get an TNS 12224: no listener error. Howver when I tnsping using the same connect string it comes back as OK. There are 3 different SID's on the box i',m connecting from (7.3.4, 8.0.5, 8.1.7). When I connect using either a 7.3.4 or a 8.1.7 environment to the same remote database it works fine - the problem is only occuring with an 8.0.5 environment.
I've pasted an example below:
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wwcrx03:/home/oracle::39%> sqlplus rman@wwtsm01_RMAN01

SQL*Plus: Release 8.0.5.0.0 - Production on Wed Aug 7 11:32:30 2002

(c) Copyright 1998 Oracle Corporation. All rights reserved.

ERROR:
ORA-12224: TNS:no listener

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wwcrx03:/home/oracle::39%> tnsping wwtsm01_RMAN01

TNS Ping Utility for IBM/AIX RISC System/6000: Version 8.0.5.0.0 - Production on
07-AUG-02 11:32:42

(c) Copyright 1997 Oracle Corporation. All rights reserved.

Attempting to contact (ADDRESS=(PROTOCOL=TCP)(HOST=wwtsm01b)(PORT=1521))
OK (1000 msec)