How to Solve Micro-master 440 Fault F090

Q:

I have a trouble with a Siemens Micro-master 440 AC converter equipped with the encoder module. Sometimes, not regularly, the axis stuck giving the fault F090 (encoder loss) but everything seems to be OK. I've checked cables, connections, encoder, increased the thresholds and so on, but the problem still appears. Anyone have kind of experience with such trouble?

A:
If has been working for years, it must to be a hardware problem. I would suggest the following:
* alignment of encoder and confirm is properly mounted and tighten to the shaft;
* the encoder connector ( wiring/soldering) in good shape;
* would consider to replace the encoder;
* can also monitor the square wave feedback from the encoder with oscilloscope and confirm is good.