Can't Paste From Clipboard Into Transaction Memo Field

I don't have an answer based on the description, but I will make these comments.

1. Not likely the problem is the keyboard settings since @estevehannon is able to paste into the memo sometimes.
2. If Quicken ran out of memory it would crash. Quicken has had bugs in the past where it was leaking memory and ran out of memory, that was the result.
3. Quicken definitely caches register data in memory. This is the reason that the first time you open a register in a new session it takes longer than when you do it a second or more times. But the amount of memory used is nowhere near the limit that Quicken has available to it.
4. It is a good practice to shutdown Quicken when you aren't using it. Some people tend to keep all their applications up all the time. In a perfect world that might be fine, but what is happening is a program is going through many different states, and it is possible for it over time to get into states that weren't planned for and restarting clears that state. This goes for Windows too, that is why restarting your computer "fixes" things. Also, it is possible people are putting their computer into sleep or suspend mode. This is another area where the computer/Quicken might not behave as expected when it comes out of this mode. But given that @estevehannon said that restarting Quicken didn't fix the problem. I would have to assume this isn't the problem.
5. The fact, that restarting the computer fixes the problem (at least for some times) suggests the problem is deeper than just Quicken.
6. And last, I will say that because of the way the registers are setup, it is more complex than it looks like. You will notice that Quicken overrides the normal context menus and shortcuts. And the only way to paste into the register is with Ctr+V. This might be part of the problem (contention of what a given shortcut should do). But I don't think that is the case in the Bills area, so again this points to something more low level, even at the Windows level.

That is interesting that Quicken's About is now showing the actual system resources of the machine now. (It used to show what the Quicken process actually had access to.) (There is also a new "More info…" that is pretty interesting)

But this isn't the memory that is available to Quicken. Quicken is a 32-bit application. A 32-bit number and therefore the max memory address, is 4GB. And in fact, because of the way memory gets mapped into a 32-bit application, it typically restricted to 2GB.

This isn't really a problem though, because Quicken doesn't use that much memory. I just clicked through all of my active accounts to get them cached in memory and Quicken is only using 240MB.

On the other hand, other programs like web browsers tend to be memory hogs. I could certainly see something like if you had lots of memory hogging processes running, that they might affect the behavior of pasting into Quicken. And if that is the case certainly either adding more memory or restarting Quicken where there isn't as many processes running might affect this.

Note this can also be a quirk/bug in Quicken, where they are using some old API or such that does get affected by these external factors where you wouldn't see it in other applications. In fact, Quicken overrides a lot of default Windows system GUI like the context menu and the main system window, and that might be part of the problem.

Good information, I wasn't aware of that. That might be another thing to try if it stops working.