GW C05F Error on Search

I have recently upgraded a GroupWise system from 5.2.3 to 5.5.4 which went very smoothly. I have a couple of users that have a recurring problem when using the search facility (they are now using the 5.5.4 client). They get a C05F error when doing asearch. I found TID 10056457 which fixes the problem but then error comes back again a few days later (it appears that something is getting corrupted). I found another TID which suggested a fix was to access the Post Office in direct mode rather than client/server – this did not seem to make a difference. Has anyone seen this problem before and any ideas on how to solve?