The story was still not at an end, unfortunately. Today, when I restarted my machines, the problem was back. Ubuntu is able to connect, Zorin OS is not. So I went absolutely under the hood, like @Aravisian 's analysis, to see what is occurring. I used the following command to debug Remmina:
G_MESSAGES_PREFIXED=all G_MESSAGES_DEBUG=all remmina
I could always see the following line in the console:
[ERROR][com.freerdp.core.connection] - Timeout waiting for activation
A google search led me to believe that FreeRDP does not work for certain versions. I left Remmina alone and tried to connect to RDP using FreeRDP itself from the command line after installing FreeRDP manually using this link:
xfreerdp /f /u :Tester /p :1234 /v :192.168.0.101
I got the same error. Ubuntu however, was able to connect. So I checked the versions of FreeRDP in both laptops:
Ubuntu gave 2.2.0, Zorin OS gave 2.3.2.
Then I started searching for how to downgrade the version. First, I uninstalled two packages, freerdp2-x11 and freerdp2-shadow-x11 which I had installed in the previous step.
sudo apt-get remove freerdp2-x11
sudo apt-get remove freerdp2-shadow-x11
Then I checked for the versions which I had available in the repo:
apt-cache policy freerdp2-x11
This gave the following output:
500 Index of /remmina-ppa-team/remmina-next/ubuntu focal/main amd64 Packages
500 Index of /ubuntu/ focal-updates/universe amd64 Packages
500 Index of /ubuntu focal-security/universe amd64 Packages
500 Index of /ubuntu/ focal/universe amd64 Packages
I saw that the 2.2.0 version is available. So, I installed the same using the following:
sudo apt-get install freerdp2-x11=2.2.0+dfsg1-0ubuntu0.20.04.1 -V
Similar for freerdp2-shadow-x11:
sudo apt-get install freerdp2-shadow-x11=2.2.0+dfsg1-0ubuntu0.20.04.1 -V
After this, when I tried to connect to the RDP, it worked!! However, still, it's a hit-and-miss, it sometimes works, sometimes doesn't. But still, it's started working, and that's all that matters. Just posting this here in case it's helpful to anyone else.