After a brief search on Metalink (no reports installation here), this error could be caused by one of several conditions - depending on the version of reports.
Apparently, under certain circumstances, it can be caused by a divide by zero, it could also be caused by a value that is too long for a column, and then I came across this note which may or may not be relevant (sorry about the formatting, its a paste)...,
Doc ID: Note:1042188.6
Subject: REP-01401 WHEN RUNNING A 2.5 REPORT WITH FORMULA COLUMN
Content Type: TEXT/PLAIN
Creation Date: 04-FEB-1998
Last Revision Date: 18-MAR-1998
Running a V2.5 report with a formula column causes REP-01401.
REP-01401 - '': Unhandled PL/SQL exception.
For example, the formula column code is:
If :JOB = 'MANAGER'
then :CP_1 := 'MANAGEMENT' (PLACEHOLDER COLUMN TO DISPLAY STRING)
else :CP_1 := 'OTHER'
Search words: REP-1401
Solution: CHANGE THE TRIGGER FOR THE FORMULA COLUMN AS FOLLOWS
This is a known issue (Bug 473584). Reports 2.5 uses PL/SQL Version 1 and
Reports 3.0 uses PL/SQL Version 2. In PL/SQL Version 2, a value must be
returned. You need to change the above mentioned example to:
If :JOB = 'MANAGER' then
return ('MANAGEMENT'); (MANAGEMENT WILL BE RETURNED FOR CF_1)
return ('OTHER'); (OTHER WILL BE RETURNED FOR CF_1)
This will resolve the error and allow the report to be run.