Just to be extra sure, I changed my Default HTML Editor Mode to "HTML Source" however the error still occurs.

Well, it is not reproducable.

You'd create a new installation from scratch into a different directory, or just in XAMPP.
That's done in a few minutes.

Joffm

Just created a new installation, same server, same version of LimeSurvey.
Uploaded the sample survey, and got the same issue.

My LimeSurvey installation is installed through Installatron, on PHP Version 7.3.33 and MySQL Version 10.3.34-MariaDB.

I just tried the same in a fresh installation of version 3.1.1 and the problem disappears in that version. 

I recommended to use a newer version. You had 5.3.2.
The demo is 5.36. The last community version ready for download is 5.3.7.

Do you maintain a complete webserver or do you use a hosting offer?
Sometimes there is a protection layer on webhosting or PHP settings are preventing correct editing and saving of questions in LimeSurvey.

Hi jelo,
I have tried a different version just to see whether the issue would disappear. I am with a webhost, using Cpanel. 
Using Installatron I have tried the following versions:

5.3.2 - Error appears
4.5.0 - Error appears
4.3.0 - Error does not appear, but I am getting different errors, relating to empty error codes (there are no empty codes), and a 403 Forbidden error when the question did save successfully
4.2.0 - Error does not appear, but I am getting different errors, relating to empty error codes (there are no empty codes), and a 403 Forbidden error when the question did save successfully
4.1.14 - Error does not appear, but I am getting different errors, relating to empty error codes (there are no empty codes), and a 403 Forbidden error when the question did save successfully
3.28.1 - Error does not appear
3.1.1 - Error does not appear

Are you saying I should upgrade from 5.3.2? Noting that Installatron does not allow me to upgrade beyond my current version 5.3.2.

Using Installatron I have tried the following versions:

Okay, then we probably are out.
These installers mostly have a customized versions that may cause problems.

Joffm
 

When using such an comfortable controlpanel like cPanel you don't need Installatron to install LimeSurvey.
You can learn to create a mysql database and upload a archive to the webspace and extract it there. The installer of Limesurvey is very easy to use and cost only a few minutes. I don't think Installatron is the source of these issues. So you might wait till you can get the latest version of LimeSurvey via Installatron.

Since you are with a webhost with cPanel, you might have issues caused by ModSecurity WAF or other PHP settings. Sometimes Javascripts get blocked by certain rules. You might find a "ModSecurity" option in your cPanel GUI in the Security section and can deactivate it for testing out.

Just self-installed Version 5.3.7+220328, and the same error appears. I have checked with my host, and they cannot see any errors appearing in Modsecurity from my IP address. Any other ideas of what I could check?

What hosting provider and product is used?

Did you check with different browsers? Without any thirdparty addons?

My hosting company is Synergy Wholesale:

synergywholesale.com/

They don't really have hosting products as you can build your packages to your needs.
My LimeSurvey runs on a package with CPU 400%, Memory 4GB and Disk Space 5GB.

It uses Cpanel, with the following options:

cPanel Version 100.0 (build 12)
Apache Version 2.4.53
PHP Version 7.3.33
MySQL Version 10.3.34-MariaDB
Architecture x86_64
Operating System linux
Perl Version 5.16.3
Kernel Version 3.10.0-962.3.2.lve1.5.63.el7.x86_64

PHP packages running:
bcmath
dom
fileinfo
gd
imap
intl
ioncube_loader
json
ldap
mbstring
mcrypt
mysqlnd
nd_mysqli
nd_pdo_mysql
pdo
pdo_sqlite
phar
posix
soap
sockets
tidy
timezonedb
uploadprogress
wddx
xmlreader
xmlrpc
xmlwriter
xsl
zip

The same behaviour is visible in the following browsers, and I haven't tested any others. Every time I have tested this has been in an incognito window which means plugins are disabled.

Google Chrome Version 99.0.4844.84 (Official Build) (64-bit)
Microsoft Edge Version 100.0.1185.29 (Official build) (64-bit)
Opera Version:85.0.4341.47
Firefox 98.0.2 (64-bit)

I too, am seeing the same error, and I am using the same version of LimeSurvey, also installed using Installatron ....

My hosting company is VentraIP in Australia, and the CPanel and other hosting specs are similar to yours.

Wow.. what are the chances! VentraIP is basically the same hosting company as Synergy Wholesale. I assume it's a setting this host has applied. 

Jelo, do you have any other ideas other than Modsecurity that we could look at?