Condensation inside flat plate

I've used a lot of AET's AE-Series and Solar Skies's SS-Series Collectors, which those Caleffi Collectors' design is based off of. (Really the exact same collectors.) Condensation inside these collectors is usually going to be one of two things:

1) The boxes are not watertight. "Neither are anyone elses'.) Especially after a few years, they can begin to take on moisture after rain, snow or heavy humidity. Normal condensation will typically evaporate after and hour or two of sunlight, with minor effects on production. I wouldn't be too worried about this unless it's staying all day and having a dramatic effect on system production.
You could try things like replacing the glazing gasket, but the backsheet and the mitered framewalls of those collectors take on water too and are more likely to be the cause of moisture entering the box.
If the collector box or frame is damaged and allowing water in, that's another story, it may need to be repaired or replaced. Some manufacturers well sell collector boxes without the absorber or glazing for a much cheaper price.

2) A brazing joint could have failed between the header and the riser tubes, letting heat transfer fluid (HTF) into the collector box. Depending on your HTF, you can look for signs of discoloration and sediment on those braze joints without removing the glazing (typically). Propylene glycol will usually look a turquoise color an the absorber plate/braze connection. Water will typically have a white-ish scale an sediment buildup on the joint. You should check your system pressures, fluid levels and operating characteristics to see if you've been losing any HTF. If this is the case and the collectors are still under warranty, you can typically braze them yourself and collect a labor allowance from the manufacturer or be sent a replacement collector.