This can happen because of just about any piece of software that incorrectly holds network buffers without releasing them. It just happened to me in Win7 64bit. Chrome and Firefox stopped being able to connect to any web pages, windows file sharing stopped working, and WinSCP and PuTTY both gave errors that included the words
No buffer space available. Oddly, Ubuntu 10 running under VirtualBox seemed to have no problem making new network connections - maybe it holds a number of network buffers in reserve.
To find out what software is leaking network buffers, you need to close programs until the problem goes away. So I started closing programs and trying the WinSCP connection after each program I closed, but the error persisted. Once I'd closed every visible program, I opened Windows Task Manager with Ctrl-Shift-Esc and started killing invisible programs with the
End Process button. Be careful - killing some things the system relies on can cause problems, so don't kill anything you don't recognize without researching what that thing is. To help identify what things are, go to
View > Select Columns and choose to show
Image Path Name and
Command Line. In general, be cautious about killing anything with a
Command Line that begins with
C:\Windows\ and be even more cautious before killing anything that begins with
C:\Windows\explorer.exe is fairly safe and can sometimes solve problems. For example, the extensions that generate thumbnails for third-party file types often cause problems, although not likely network-buffer-holding problems unless you've installed trojan extensions sending data back to some malicious server. Killing
C:\Windows\explorer.exe will make your taskbar disappear along with all your file explorer windows. To get them back, go to
File > New Task (Run...) and type
explorer in the box that will appear, then click
Keep ending programs until your problem is fixed and the last program you ended is likely the culprit. Sometimes ending a program will release a few network buffers that the program was legitimately holding, which may be enough to allow you to make a few successful network connections in another program using those few released network buffers. Therefore, you should double check that the problem is really solved by opening a lot of web pages or other connections at the same time.
In my case, killing fmsib.exe (part of FileMaker Server 13) let me make one new connection, but no more. Killing fmshelper.exe (also part of FileMaker Server 13) let me make dozens of additional connections, so I think it was the culprit, but that's only in my case.