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Do you have a probe offset (what type of probe is it ...)

It is a normally open aluminum block, with the parallel port running for 4 years.

Ok, given it worked fine (4 years with a parrallel port?) at maybe 25khz kernel speed I suspect that you have installed the UC300ETH and selected a kernel speed that is too high for your drives so your drives are missing pulses (NOT LOOSING STEPS) but missing pulses because the pulses are being output too fast for your Break Out Board (BoB) or your drives to see.

Please note I do not run Mach3 with the UC300ETH and have never loaded the plugin for Mach3 + UC300ETH so I suspect (think) that possibly there is a kernel speed setting within the Mach3 plugin that is set too high. (I tried to load the Mach3 plugin, but I do not have a spare UC300ETH at my laptop (where I am now))

If the kernel speed is set to 400khz ot 300khz, try setting it to 100khz and trying again..... most drives and break out boards will run fine with a 100khz kernel speed setting and as you were running an LPT port in Mach3 fine, it was probably running a kernel speed at 25kHZ.

Yes, but now I have installed servomotors

Ok so its actually not been working for 4 years!!!... and the only thing you changed was from LPT (parallel port) to UC300ETH....

Seldom ever get the full story.....

The servo motors.... have you set them up correctly?? Have you looked at what the allowable following error is?

Again.... most servo motors these days just take a step and direction signal and for all intense & purposes they are the same as stepper motors [yes they have encoders and run closed loop etc]....
Suggestion If you are 100% certain that your macro works (for 4 years) and all you have changed is the UC300ETH from LPT and your stepper motors to servo motors.... do you maybe not think that maybe you've not set up the system right?...

If you switched it back to LPT (which I thought you may have done), do you not get the same inconsistency ?

Why don't you post your profile ( profilename_.XML ) from Mach3 and someone will maybe take a look.

Note that this forum is mainly for UCCNC.... there is not a lot of traffic for Mach3/4 through here and maybe your question(s) may have more help via the Mach support Forum.

How certain (100%... 95%.... .... 5% ? )are you that your servos have no following errors or the following error is so small that when the skip signal is tripped the drive stops in exactly the right location?

I am not a servo user (I have 1 for a spindle on a lathe and that is it).

The machine works perfectly, these are some machined parts, the probe for some reason is not good

I'm sure everything is ok but for some reason I do not know the probe does not always measure the same

You've really not given us any information at all to have an idea what is wrong.
Pictures of your probe setup?
Detailed info on the errors you are seeing?

I have mach3 version 3.043_2018 with windows 10 and uc300eth with two UCBB dual port breakout boards. The probe is connected normally open to pin 15 and gnd of port three, the first measurement is always wrong 6/10 after it is correct. I tried to change the probing speed, the macro, the version of mach3 but the problem remains,
when I put a capacitor between pin 15 and gnd the error of the first measurement is smaller 4/10. The whole electrical part of the milling machine is ok. some idea?

You say it's wrong.
How much is it wrong? What are the values you are seeing?
Did you try what I suggested earlier, removing the retract?