error 207930: "Z1 Drive: Brake control error status (31 or 32 or 40)".

Hello.

I place in operation the machine tool, Sinumerik 840D sl and 7 axes. 4 axes are connected to NCU720, 3 axes are connected to NX15. I have two engines with a brake in my system. One engine with a brake is connected to NCU, other engine with a brake is connected to NX15. I have no problems with the engine which is connected to NCU. I periodically have a problem with the engine which is connected to NX15. It is a problem arises only at the first inclusion of drives. There is an error:

207930: Z1 Drive: Brake control error status (31 or 32 or 40).

This error appears even then when the drive has no permission. There are no signals DB3x. DBX2.1 and DB3x. DBX21.7.

Why the system gives out such error periodically?

How will get rid of such error?

How to solve this problem?

I checked wires.

After button click "Reset" in MCP the error disappears and having given permissions it is possible to operate the engine. Sometimes it is necessary to press the button reser some times (10-20).

Thanks.

TIA