Sure PLC canceled your spindle enable during it running you need find reason --I attached Siemens alarm explanation 

MrPiNG

Hi,

Now I got this error (first time I think). The 611U is still in RUN mode on all 3 servodrives. My spindle is registering the encoder signal atleast and a RESET on the MCP fixed the problem.

Is this coming from the PLC ?

MrPiNG

What would be reasonable here ?

Please check PLC logic, drive pulse enable signal DB3x.DBX93.7 is not set or reset during motion which triggers alarm 21612. Here x = S1/C1 drive number in configuration.

Your last picture remind me. Do you know the Ter.65A/65B --control enable , there are actually 2 tiny relays on the 611U card. I have couple of customer services cases , which the tiny relay no good.

Ter. 663 is the same but one controlling A and B axis 

If you use brand new 611 card , the problem still happen , you need to check PLC program to see some external fact to cut off the enable. such as temperature  sensor, limit switch , speed monitoring sensor. they normally connected to Input , as condition of spindle enable. you need read PLC program 

 I made red line to point a relay, which is possible "663" for 65 A/B I cannot find from your pic.

However I don't suggest you replace tiny relay , but you can try this "663" one. very careful using special soldering device , or have it done by professional electronic repair shop  

I have worked for siemens CNC and drive from 1989. but mostly focus on module basis. I have patterners who repair what I told them. I am 100% sure there are tiny relays on 611A spindle control  card (1991 to 2002)   and tiny relays one SINUMERK 840C analog output card.

For small relay (large than tiny, like "663" ones) There are one 611A axis control card and 611D cards as well. that is not Siemens official conclusion, just from myself services experiences  

MrPiNG

Can you show me that relay on the board pictured above ?