Cannot take screenshots...

[Kirika-]

In the Windows Search slot, type "Controlled" and select the result "Controlled folder access" and click it.
Turn off Controlled folder access.
See if the screen shot function returns to normal.
If it does, great, leave the Controlled folder access switch off.
If you would rather still use the anti-ransomware feature, then have the switch on and use the "Allow an app through Controlled folder access" link, click the + box for "Add an allowed app", then navigate to the FF14 game client install location and add the game .EXE file.

The anti-ransomware feature was added to Win10 in the past year or two, but Microsoft is slow to add "default whitelisted" programs to it's built in definitions, so it's hit-or-miss as to if the exact FF14 you have is pre-OK'd. This is why not everyone has the issue with screen shot saving function, as they either have an updated whitelist, custom set their FF14 screen shot folder outside of Window's protected folders, don't run the Windows protection, or have a game client version that was OK'd.

If you can't adjust the settings on the protection, then your windows user account truly isn't at Administrator permission levels, which will also result in the screenshot function of the game to not work.

[ICountFrom0]

In the Windows Search slot, type "Controlled" and select the result "Controlled folder access" and click it.
Turn off Controlled folder access.
See if the screen shot function returns to normal.
If it does, great, leave the Controlled folder access switch off.
If you would rather still use the anti-ransomware feature, then have the switch on and use the "Allow an app through Controlled folder access" link, click the + box for "Add an allowed app", then navigate to the FF14 game client install location and add the game .EXE file.

The anti-ransomware feature was added to Win10 in the past year or two, but Microsoft is slow to add "default whitelisted" programs to it's built in definitions, so it's hit-or-miss as to if the exact FF14 you have is pre-OK'd. This is why not everyone has the issue with screen shot saving function, as they either have an updated whitelist, custom set their FF14 screen shot folder outside of Window's protected folders, don't run the Windows protection, or have a game client version that was OK'd.

If you can't adjust the settings on the protection, then your windows user account truly isn't at Administrator permission levels, which will also result in the screenshot function of the game to not work.

Amazing that it takes you, saying this, to fix the problem (at least, we HOPE it does), and nobody on staff actually had a clue.

[Kirika-]

Amazing that it takes you, saying this, to fix the problem (at least, we HOPE it does), and nobody on staff actually had a clue.

While it might not fix it for everyone (as the Windows user account settings and file/folder security permissions can cause this issue too), it is a common issue with games that write to the user folders and Windows Ransomware protection.
Disabling/white-listing can also fix the issues of the DXclient EXE file(s) in FF14's engine and the games timed "auto-save" for the character settings, where you will get an in-game chat box message saying to the effect "such-and-such file can not write/save to such-and-such" in red text.

Also, it seems about every time Windows' built in anti-malware is updated, it has a extremely small chance of messing up it's "friendliness" with FF14's game executables, which then triggers the rare "it was working fine but suddenly its not..." issues.