I’m visiting Ranua at the end of August 2019 for a few days. Are mosquitoes a problem at this time of year?
Not from a local point of view. There might be a few, but nothing a long-sleeved shirt or repellants wouldn't handle.
This is a bit nitpicking, I know, but late autumn mosquitoes are seldom a problem but a different bug called "polttiainen". Polttiainen is much smaller than a mosquitoe but equally annoying. But late August not a big problem.