Is this sentence correct?
We had published the OpenCCU source code at the beginning of 2002. At first we had uploaded source code on Sourceforge and immediately began receiving thanks and wishlist items for the future OpenCCU development.
The whole narration is in the past tense and the publication of the source code and subsequent feedback precedes other events.
Using the pluperfect makes the reader expect that it is "leading up" to a statement subsequently narrated in the preterite (or potentially other past tenses). So long as that's the case, you can have multiple sentences in the pluperfect as you have. Otherwise, the use of the pluperfect is odd.
The way your sentences are written makes the past perfect unnecessary here. The actions described take place one after the other, which makes the use of the past simple appropriate. If you change the order in which you mention the events, then you can use different tense sequence:
We published the OpenCCU source code at the beginning of 2002. We began receiving thanks and wishlist items for the future OpenCCU development as soon as we had uploaded source code on Sourceforge.
If you want to tell what happened some time in the past, use "past tense".
We published the OpenCCU source code at the beginning of 2002.
if you want to tell what had happened before something happened in the past, use "past perfect and past tense" combination.
We began receiving thanks and wishlist items for the future OpenCCU development as soon as we had uploaded source code on Sourceforge.
Since "the publication of the source code and subsequent feedback precedes other events", I would like to see more context to make a judgement. But, based on that fact, I would say that use of the past perfect is indicated.
The first thing I notice (other than the fact that it's a paragraph, not a sentence?) is the alternate possible meanings for the second word in the first sentence. The use of had here allows for different possible meanings. The first is probably most easily understood as "We'd", meaning that whoever we is actually did the publishing. The second may be understood as synonymous with the phrase "We'd arranged to have published...", meaning that is was someone else who did the publishing. So if your version of "correct" is that it affords as little ambiguity as possible, I suspect you'd class that aspect of the first sentence as incorrect. It appears a more preferable initial sentence would be
We published our OpenCCU source code at the beginning of 2002.
The second sentence also flirts with the word had in what may be perceived as a similarly problematic way. But it's the use of the word on there instead of the word onto which allows for an even larger amount of ambiguity, (though it's easy to proclaim that ambiguity irrelevant depending on any number of possible contextual factors). If Sourceforge was the destination, as it no doubt was, the word onto makes that incontrovertibly clear, while the word on allows Sourceforge to be the place from which the uploading took place.
From there, if you don't mind the critique, the largest problem centers around the way that the second sentence begins, that is, the use of the term "At first". This of course is synonymous with the adverb initially, modifying the verb "we had uploaded". The reason this is problematic is that by sheer dint of use (inclusion) it implies a contrast between that moment and another moment at some point in the future, near or far. It implies an "And then later...". Unfortunately, the next adverb immediately fails quite understandably to accomplish that contrast. It appears that what the second sentence intends to communicate is something more along the lines of
And the instant we uploaded our source code onto Sourceforge we began receiving thanks as well as wishlist items for the future OpenCCU development.
Finally, the phrase "for the future OpenCCU development" is also perhaps less effective than it could be, though certainly most readers would understand that it apprehends work that may be done on items from the aforementioned wishlist. Largely it is the use there of the preposition for which accounts for the drag, because for can also mean on behalf of or on trade, etc. Cleaning that up seems appropriate. And frankly, the very concept of a wishlist implies not only the future but, indeed, the need for development. And the fact that OpenCCU would accomplish that development is also sufficiently provided for by context alone. So it can be excluded outright.
In sum then, the following construction appears more suitable
We published our OpenCCU source code at the beginning of 2002, and the instant we uploaded it onto Sourceforge we began receiving thanks as well as wishlist items.
