How can I check that out??

Word - No Lock
AES - Sync Lcok 48khz
ADAT 1 - No Lock
ADAT 2 - No Lock
MADI Opt. - No Lock
Coaxial  - No Lock

This is the current state of my input status.
Is it correct that my main clock is RME UFX+?
I wonder where I can see the main clock and where I can change it.

Checked Kemper forum, there seems to be a limitation in HW as of 2018.
But I remember I read in another forum they wanted to change that.

I also read that Kemper customer support wouldn't answer such things on forum. So you should ask them, whether its possible for the kemper to get clock from the recording interface.

This would have the advantage to you, to use UFX+ as clock master.
Then you would only need to set the clock in the DAW project and all other devices, that are digitally connected to the UFX+ could get the clock through a digital input, if they have.

Otherwise you need to use the Kemper as master clock. UFX+ steadyclock mechanism will take of eliminating/reducing any potential jitter. Then in the UFX+ driver settings you need to set clock source to AES.

An alternative would be to add an ADI-2 Pro FS R BE to your setup, which has a built-in Sample Rate Converter (SRC).
Then Kemper and UFX+ can be clock master at the same time.

It even would have the advantage to be able to use the Kemper with maximum sample rate fully independend on the sample rate which you use in your DAW project to record.

Kemper <-----------------------> ADI-2 Pro <---------------------> UFX+
(SPDIF)                                                      (AES/ADAT/SPDIF)
44.1-96 kHz                                                                      44.1-192 kHz
[Clock master--------Clock Slave|SRC|Clock Slave.............Clock Master]

You could also leave out special adapters/converter, also in your current setup.
The manual tells you about an AES/SPDIF Adapter, which most likely would have been even cheaper.
Here in this proposed solution with the ADI-2 Pro FS R BE you would only need standard SPDIF / TOSLINK cables
and would have additional advantages of the ADI-2 Pro FS for monitoring through monitors / headphones.

> Then, if I set the clock source in the clock mode as Internal, can I say that UFX+ is being used as the main clock?

I can't answer this question because you quoted everything and didn't tell to which of the cases you refer now.

EDITED

Please ask Kemper Support whether the Kemper can be configured to get clock from it's SPDIF in.
If this is still not possible then the Kemper will act as clock master.

As a consequence of this you can't set the UFX+ to act as clock master.
There can be only one clock master in such a setup or the digital links won't synchronize to each other.
As a result of this you will have audio glitches and dropouts.

Using the Kemper as clock master is not tragic from pure functional perspective. You do not even have to be worried about things like clock jitter, SteadyClock (TM) will take care of this.

From operational perspective it would simply be much easier and straightforward if the UFX+ could act as clock master, because then you could control the sample rate of your environment by the project properties in your DAW.
Then you would only have one location where the sample rate is being set.
And in case you work with different sample rates then it would be easier, after opening projects in your DAW that might have different sample rates, that the rest of the environment (UFX+ and Kemper) would follow that rate without having to reconfigure any other device in the "chain" / "digital domain".

The best most flexible solution (not cheap) would be to get additionally an ADI-2 Pro FS R BE as I told you. Then you would at the same time improve your monitoring section with the ADI-2 Pro in general, Kemper AND UFX+ can be clock master and you control the sample rate simply by e.g. opening your DAW project or running Music through your player with ASIO support like e.g. MusicBee. The Kemper then can run then instantly at e.g. 192 kHz no matter what you set in the DAW project, there you could even record with 44.1 kHz, the SRC of the ADI-2 Pro would automatically downsample it to 44.1.