JasonPrince

Yes, two reasons for that. The one is that when putting different-sized wires side by side into the same hole, it is easy to not sufficiently clamp one of them because of the difference in size. Additionally, hot metal expands and contracts, so such wires tend to "wiggle" themselves loose unless you properly torque the cr*p out of them right from the start.

Now take a look at this (Jaco shared some pics offline, in case you wonder how I got this)...

Those wires in the red circle are AC wiring then. That cable indicated by the arrow is a DC cable... a very hot one, one that can melt insulation. They run side by side into the trunking above.
As my father would say: Mooi man!

yeah that looks like 10mm^2. That's underrated. Even 16mm^2 would be too thin. 25mm^2 would be bare minimum, and 35 is definitely advised.
Better idea. Load VictronConnect, and set the maximum limit of the solar charger(s) to 50A. Then you can at least utilise some of the power until the cables can be replaced.
Disclaimer: Did not read the whole thread. Shooting from the hip.

At the bottom right of the MPPT, you will see a little black wire in a green Connector, pull that connector out and the MPPT, will not produce any power till you push it back. That should also safeguard your system for now. I am also worried about the power cable into the inverter.  Seems like a 4 core surfix, I just hope its 6mm 4 core... 

Jason,
Surely this should be classified as an emergency and warrants immediate attention despite the lock-down. I had a dangerous situation at home and managed to get an electrician out. Neither @Jaco de Jongh nor @Soltaro are trying to be alarmist, just brutally honest.
Good luck.
 

4 strings, I would have run 4 mm per string and then after combining the 4 I would have used 16mm (4 x 4) 
 
Definitely yes, that is why a mentioned, if you can disconnect at least 2 of the strings until they can fix the issue , you would minimize the risk.
Please tell your installer, he does not need confirmation from Victron, the SANS regulations tells you how much current is allowed for each cable size and for 16mm it is 60 Amps, 25mm = 80 amps and 35mm 100 amps, (Rule of thumb and depending on installation method). He is trying to do almost double that. 
EDIT: I have PM'd you my Cell number, You and your supplier is more than welcome to phone me. 

Going trough halve of a compression gland, that is max 16mm in my opinion (I suspect less). A 25mm cable will basically fill up the hole of the compression gland, the 35mm will go through very tightly. 
That cables are way too thin. The PV looks like 10mm and at full production, they should create  a bit of heat themselves as well. 
That might point to a loose connection as well, but the cable might be contributing. 
EDIT: Just went and checked in my Vehicle. Blue Welding cable is 25mm, look as the difference between the 25mm cable (Blue) and the other cables. I am afraid, you stand a serious risk of fire, they need to sort this out soon.. 
If you want to be safe and the combiner box has fuses per string, pull the fuse on at least 2 strings. 

Sounds like they installed under rated cable there. Post a photo of wiring running from the MPPT and inverter and batteries @JasonPrince. For that size MPPT and Inverter your need to look at 35mm2 cable and maybe a bit bigger.   

This is not normal at all. You have a 250/100 MPPT, with 5440whp on the roof, so in perfect conditions, losses ignored, you can expect 5440/53 = 102 amps, but the MPPT will clip the top off and allow 100 Amps. To handle that you would need 35mm square cable at least. Anything less than that will overheat. 
Made the mistake myself to install 25mm in on those MPPTs because the software tells me the cable can handle that, but in practice its not working like that. In one situation, i installed 25mm on a 250/100 and it was running at 89 degree C, changed it to 35mm and the temp dropped to 57 degree C.