Are you absolutely sure that it's a viral flare and not just an ME symptom flare? B2 made my ME symptoms flare when I tried it a few years back. I didn't have any viral testing done, so I can't say that it wasn't a viral flare, but I had no reason to believe that it was one. I assumed that the B2 was just affecting some process in my ME. B3 had a similar, but stronger effect, and added suicidal moods as well. B12 had the same effect as B3 (including strong suicidal moods), but only at that particular time. I had a B12 shot years before that, which didn't affect my ME, and I tried the same B12 tablets many months later, with no significant effect, so the B12 issues seem dependent on some other factor.
B1 seemed to give a slight benefit the first time I tried it, but never had a significant effect after that.
So, the effects of B vitamins seem to be complex, and they may not be linked to viral infections. My personal guess is that it's not causing a viral flare in you. If B2 supplements aren't providing a noticeable benefit for you, I suggest not taking it as a supplement, and even avoiding B2-enriched foods.