We've just bought an APC Smart-UPS X 150VA and were surprised with how frequently (many times per hour) does it switch to battery just to switch back to line in about a second. The light doesn't blink, nobody is arc-welding around, there are no problems if we don't use the UPC, the voltage is ok (235+/-1V, according to the UPS indication). Why would it do that? Is it ok (won't it wear-out too soon or even explode?)? Can this be configured (sensetivity of some sort, maybe)?
You're probably being misled a bit by the UPS display versus what it's actually measuring inside. The UPS is most likely responding to transient (under a second) over/undervolt conditions. Since the instance is under a second, it isn't being reflected on the front panel, which probably only refreshes every second.
Frequent switches to battery does wear the battery down somewhat, so you'll probably have to replace the battery sooner than you'd expect with otherwise clean power.
It has been a long time since I've seen a SmartUPS interface, but for the higher units (last I dealt with was a 3KVA unit ~8 years ago) there is some tunability for sensitivity. I don't know if that functionality is present in the control panels for the smaller units.
This is usually a result of the device detecting a fluctuation in the line supplying it with power. (And during self tests, but those don't occur frequently - every couple of weeks for most models.)
From the manual:
(EDIT: I was going to post the manual, but it seems my link is for a 1500 model, not a 150 model. Is that a typo in your question?)
So, actually, this causes extra wear on the UPS (which is probably better than extra wear on the equipment plugged into it), rather than preventing it. And yes there is a Voltage Sensitivity setting you can adjust on your device.
As to why you don't see it on the front panel, that would likely be because the fluctuations are shorter than the LCD refresh rate. If you want to be sure, you'll need to actually measure the power coming in with a lot higher resolution than the UPS displays. We get a lot of sub-second power fluctuations where I am, and the power's generally pretty "dirty" anyway, but it still requires proper equipment to measure and graph - you can't do it by relying on some LCD display on a device that's not actually intended for that purpose.
I'd go with some kind of a waveform problem - either something really far from sinusoidal or wandering frequency. The best way to check would be to use a power quality meter or, alternately, an oscilliscope (with an isolation transformer!).
The symptom you describe is precisely what I've seen when using cheaper generators. Some units - usually higher end online devices - can tolerate some pretty awful power, but a line interactive unit may have a hard time figuring out what to do. In practice these units are often more sensitive than a standard switching power supply (i.e. the one on your server), which can happily keep making DC with some pretty awful waveforms.
Can this damage the UPS? Over time, definitely. I wouldn't leave it hooked up. Ideally you should get the power fixed. If that isn't doable then look into online UPS units that are OK with dirty power. These units are generally preferred for any kind of professional setup anyhow.