Can intentionally omitting related work (citations) be considered as misconduct?

Yes. If they were indeed aware of the fact that a paper exists which proves that the problem was solved, then they clearly commited a scientific misconduct, though less so, I should think, by not citing your paper, and more so by claiming that a problem had no solution which they knew was solved already. Even if they considered your proof wrong, they should absolutely make it clear why they disagree with you.

However, as commenters have pointed out, there is always a good chance that people don't realise what a paper is about, don't remember that there is that paper, or they wanted to discuss your paper and just forgot about it, which of course doesn't speak for their academic diligence, but means that no misconduct was commited (unless your paper is so high-profile that by not including it, they are neglecting their duty to read the literature before writing a paper.)

Edit: Note that I have answered the general question in your title, not the more precise question in your text, which I don't know how to answer without additional evidence.

Another edit: The formal rules for scientific integrity of my own university, for example, do not explicitly mention omitting a citation, but do (obviously) contain "intentional or grossly negligent misrepresentations in an academically relevant context" (my translaton) as an act of misconduct. This is exactly my point: If they knew, or really should have known, it's academic misconduct. If they didn't know and your paper is so obscure that they didn't act grossly negligent in not finding it, it isn't.

I'd suggest contacting the editor of the journal that published their paper. I'd also suggest not insisting on scientific misconduct in the early stages of the communication.

If the authors solved a problem that has exactly one prior solution (yours) and failed to reference your paper, it is a very peculiar situation. Lets leave the authors aside for the moment. That kind of oversight should be caught at the review stage (it is after all a rather quick search and the reviewers are supposed to be experts on the field), so that leads me to question the reputability of the journal. On the other hand, it simultaneously leads me to question the reputability of the journal/conference where your work is published. Many authors don't read articles from predatory journals/conferences.

If the above is cleared in a satisfactory manner, i.e. none of the involved journals are predatory, we can get back to the authors again. Such an oversight is indeed probable cause for a poor literature review at the least. One possible caveat would be that the problem they solved isn't exactly the problem you solved, but some variety or special case. That may be hairsplitting of course, but proving misconduct is an extremely delicate matter.

I hope that it is now clear why you should raise the editor's attention but refrain from making strict accusations. If you are right, a reputable journal will (in the best case for you) retract their paper. Once this is accomplished, if you still feel insistent, you could further pursue the matter of scientific misconduct with the authors' institution(s).