mastering error in external axis

are all axes connected?
are they connected correctly (no mixup)?
is second RDC connected?
is deployed project correct (matching current hardware, topology)

Define "switching of the controller". Do you mean a reboot, or a complete power off/on cycle?

The mastering error occurred only for E1-E3? That is unusual. Is the KP3 an old unit? Possibly recycled from a KRC2? Where is the secondary RDC card for the KP3 located? I would check that first.

It might simply be a one-time fluke -- you can re-master the KP3 axes easily enough. However, if it keeps happening every time the KRC4 is rebooted, then the secondary RDC is definitely in doubt.

[size=1em]"Everything is connected" is NO guarantee that all connections are really correct and that they match work visual project.
Ever seen idiocracy movie?[/size]

[size=1em]Just like in a plc - you CAN assemble valid unit from mix of parts (different processor, variety of communication and IO modules). But if your plc project (software configuration) does [/size]
[size=1em]not EXACTLY match your hardware selection (uses same components, and in same ORDER), your Plc will have NO chance of working correctly....[/size]

[size=1em]Same applies to robots and WorkVisual configuration. Your project must contain [/size][size=1em]right [/size][size=1em]hardware, [/size][size=1em]right [/size][size=1em]topology for each bus etc. Simply "connecting things" is not enough.[/size]

[size=1em]One Rdc can be used for up to 8 resolvers. This is not enough to connect 6-axis robot and 3-axis positioned (6+3 > so you [/size][size=1em]must [/size][size=1em]in this case have second RDC. There are different ways to connect RDCs and they must each have valid FSoE address.[/size]

[size=1em]Next all axes must be connected to CORRECT ports of RDC. If they get mixed up, all incorrectly connected axes WILL loose mastering. This happens easily on external axes since resolver cables are individual and most people assume it is all plug and play and doing so without training is somehow supposed to still be ok. I would recommend to check motor connections are really correct and once they are - label everything. Swapped axis cables will also cause runaway axis problem and robot will fault after "buzz" or "kick" of all axes.[/size]