Sorry for that. The problem is not the G3, but the non-current 10.3 version you are using. 4.05 ran fine on a G3 with 10.3.9.
I've just fixed that; by the time you read this there should be a new App 4.06 for PPC Mac that your client should automatically download with new work. This was tested successfully on 10.3.4, which is the oldest I could get my hands on.
BM

Problems with new S5 worker on PPC G3
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No. Charlie Fenton, maintainer of the Mac port of BOINC, gave up on trying to port BOINC back to 10.2. Threads and shared memory handling isn't working for BOINC there.
BM
BM