My house has about 20 LED bulbs of 7,8 or 9 watts. Several have appeared to fail in the 15 months since new. However, I have discovered that the problem is a poor connection. If I tighten the "failed" bulb by a small amount perhaps only 10 degrees it comes back on. I have never had this problem with UK type bayonet fittings. Can the 50 Hz AC cause enough vibration to unscrew the bulb enough to disconnect it ?

Nah, it's corrosion in the humid atmosphere.

Tightening it scrapes the rust off.

LEDs get hot, not quite as much regular incandescent bulbs but the metal screw-in expands and contracts as the fittings heat and cool down with with lighting cycles. Eventually they become slightly loose and as the OP discovered, a slight turn fixes any problems. This happens with screw-in filament bulbs too. The UK bayonet fitting does give a more solid contact on the barrel side.

Frequency can have affect on AC resistive loads but not that you would notice.