Vocal playback sound not working but guitar does

I bought this as a gift for my 14 year old son last week and he is really sad right now that it is not working as intended. He has a Røde NT1-A microphone and a soundcard Steinberg UR22mkII and an acoustic guitar with a microphone in it. I run Windows 10 installed on a SSD drive and 8 GB RAM. I have put soundcard on Line 48V to activate the microphone.
I run Cubase AI 10.0.15 Build 136
I tried to add as attachement the settings with pictures in a pdf file and a video showing example how it sounds. But it is not accepting. avi, mpg4, pdf, ppt etc.
My current settings are:
Audio connections – Input – Mono 1 – Yamaha Steinberg ASIO USB -
USB 5V activated with micro USB 1A/5V.
Buffer Size 2048

When recording the guitar I get nice and steady output sound when playing it from the project zone using playback start button. It can start playing for half a second and go down dead silent for many seconds and then come back again or be dead all the time. When looking in the mixer the vocals show no output when it goes silent. It is both failing in speakers connected to Line out L/R and headphones. The failure follow when exporting file and open in media player. It can behave differently if I wait some minutes and play it again. A vocal that was silent can start to play sound the first second if I didn’t play it for some time.
I tried it on my IPAD and there it worked instantly. This makes me think that it is the settings or how my computer behaves.
It is really frustrating and hope someone have some thoughts of how to proceed.
Below are some things I have been doing to try to figure out what is going on.
I have tried to move around the soundcard to different internal slots on the computer (but no external USB-hub).
Tested to record on both SSD drive and 7200 rpm HDD
Tested both small and big buffer size.
Switched to ASIO4ALL DLL with same outcome.
Removed and de-activated my computers Realtek audio device and my computers AMD audio device.
I have checked computer settings and find no problem with it.
Switching between input mono 1 and 2 for the microphone.
Checked monitoring speaker button and put it on and off.
Reset audio settings and started all over again several times.
I have some plans to install some alternative to Cubase to see if it works. Any suggestions. I Installed FL Studio but it looks complicated to getting started to just test this issue.
I have tried to import a CD with audio and it works like a charm to play.
I’m out of ideas and wonder if this is just a beginner’s failure or if I need to buy a new computer or reinstall windows.

Thanks in advance