Cannot add another channel to MQTT thing

I am running OH 2.5.2 Release Build.
I have three external MQTT brokers, each defined as a Broker Thing. They all are on-line
To each broker thing is connected a generic MQTT thing that have channels defined, sometimes as many as 18. All generic MQTT things are on-line as well.
When trying to add another channel to one of the generic MQTT things I get the error message: Error creating channel. I am using the Paper UI.
I think (as far as I can see) that I am following the pattern of all the earlier channels, so I cannot understand what is wrong this time. I have tried clearing the cache without success.

Any idea is greatly appreciated. Where can I get more info on the error? Which log?

Make sure you are giving the new channel a unique name; Paper UI rejects duplicates.

Of course. I have checked that and tried several names.

Could you post the complete error message you get?

And just to be sure, you did create those generic things via PaperUI as well?

Yes, I did create them all via Paper UI.
The only error message I get is a small flash square in the lower right corner saying “Error adding channel” Is there a log I could enable to check?

On your dashboard you should find the logviewer.
Since this message is displayed in PaperUI, it should be coming from the UI.

Upgrading to 2.5.4 Release Build, restart, cleared cache did not really help. Still the same error message. But suddenly, after a number of attempts the channel definition went through and I could link the Thing to an Item.

Now, trying to repeat the same trick again, no luck. Same error message, no result.

Same issue here. Anyone have an idea? I got Error 500 - Internal server error. I don’t find any trace of error in the log.

update: Ok, find out my error. When adding “Channel ID”, I was trying to write the complete channel name “mqtt:topic:5629151d:sonoff_DHT22_1B5946_IP”. I just have to write the last part “sonoff_DHT22_1B5946_IP” because prefix “mqtt:topic:5629151d” is alreay include in the thing itself.