What's in the stderr output of the "Client error" Tasks you got looks pretty bad, most likely a malfunctioning CPU due to overheating. There is also a small chance that it's "just" a broken harddisk, or the main memory. Whatever causes the Client errors is also probably the reason why even the Tasks that run without crash (i.e. "successful") send back results that are found invalid. Check your machine!
BM

no granted credit at all, for several weeks
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No doubt. It depends on what you normally do with it. BOINC is actually meant to use the resources of your computer that are usually not in use, so it's no so much of a surprise that it discovers errors that don't show up in normal usage. It might be a memory fault in some segment not normally used, or a heating problem that only occurs after hours of heavy use of a certain part of the CPU core that your other programs don't touch. If you're working with pictures or movies or audio data, you may use data formats that are - to some extent - resistant to changes in single bits or bytes, so a memory fault could remain unnoticed.
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BM
RE: Well, actually... Not
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I was not referring to the BOINC Core Client (which is a program), but to BOINC as a concept of and infrastructure for volunteer computing, thus including e.g. the BOINC projects ;-)
BTW: I know some cluster admins that have Einetein@Home running in the idle time of the machines to detect faulty systems...
BM
BM