What does the error code c101b000 represent?

Thanks you @93tid for your prompt response. I will look into upgrading the phone to windows 10 and definitely wait for 48 hours. :)

I know it might sound silly but, how do I update to latest OS as I dont see the update notification and neither I can download the Windows Insider application due to the error I am facing now.

Is it that I have to wait until my 645 XL get an update by itself or is there any alternate way to force update the device?

As already stated clearly above, there is nothing wrong with your phone and nothing you do with it will resolve the problem. You just have to wait for MS to sort out the problem from their end.

This issue comes up on regular basis, as you have already observed - thank you for making the effort, unlike the dozens of others who blindly post new threads on the same page already full with the same question asked over and over again. Usually it is resolved within 48 hours or so.

You might want to consider upgrading the 640XL (there is no Lumia 645) to Windows 10 Mobile, since support for WP 8.1 ended in July 2017 and the store will be supported for around 12 months after that.

https://support.microsoft.com/en-us/help/4036480/windows-phone-8-1-end-of-support-faq

What does end of support mean for customers?

End of support means there will be no more product or security updates.  This is in-line with our Support Lifecycle policy.

Even after end of support, automatic or manual creation of new device backups for setting and some applications will continue for 3 months, ending October 11, 2017. Other services including photo uploads, restoring a device from an existing device backup and the Store will continue to work for at least another 12 months from end of support, after which time, services will start to be discontinued.

There is a problem with the Store from Microsoft's side, you just need to wait, nothing you can do with your phone will fix it.

There is a problem with the Store from Microsoft's side, you just need to wait, nothing you can do with your phone will fix it.

How often this issue comes up? I've been seeing this error in posts dated back to 2012 onward. I am currently using Lumia 645 XL Dual Sim phone (running 8.1 OS update 2) and have this c101b000error since 04/12 and its been 24 hours approx. since I encountered the error. Tired soft reset, performed factory setting, tried to change time and date manually, removed battery and restarted it again but and I dint see anything work for me.

Any advise is appreciated to fix this phone. Its actually my mum's phone and she is used to this and wont move move to another phone. So this is bit critical for me :D

There is a problem with the Store from Microsoft's side, you just need to wait, nothing you can do with your phone will fix it.

I had the same problem. After several tries I disabled "roaming" and "wi-fi". After doing this the download installed perfectly. Hope this helps.

already I have disabled roming and wi-fi , but there is no change in this problem....