I am a male and in my FTDNA DNA-matches, when I filter for matches by X-chromosome match, I get around 30 people who can only be possible relatives from my father side (my parents came from distant European countries, the matches are from the region my father was born, and one of the matches is most likely related to my paternal grandfather. Is there a possible explanation or there's some kind of mistake in the test?
FTDNA will include segments as small as 1cM in X-matches; you'll need to compare in the chromosome browser to see how much you share with these paternal matches. If these matching segments on the X chromosome turn out to be very small segments, they're most likely Identical By Chance (IBC) and can be disregarded.
There are many possible explanations.
First, make sure these are really X matches. As others have suggested, look in the chromosome browser. The filter may or may not be accurate.
Second, I agree with Leah: disregard segments that are too small. All segments are "real" but that's not the same thing as "meaningful." Anything less than 5 cM is almost always useless. For regular autosomal, most people only look at segments at least 7 cM. X recombines less often so aim for segments of at least 10 cM.
Third, your ancestors traveled around a lot more than you might think. I mean if your dad was from Scotland and your mom was from Burma, there's probably not a lot of doubt which side you match someone on. But Scotland and Romania? I wouldn't make assumptions. And with the Roman empire moving all over Europe, you get Middle-Eastern and Northern African migrants too. It's just not something where you can state "my family is from such and such country and has always been from such and such country."