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Now this is an interesting problem!  We are using the SharePoint 2013 FBA pack on a project that has multiple WFEs.  Within the registration process is the ability to use a CAPTCHA image for human validation.  The registration page looks something like this.

In our QA system (where internal customers do lots of acceptance testing) we were having an issue where the CAPTCHA picture would not render all the time.  Many times it would render as a blank HTML picture box.  I ended up testing each WFE server in QA and found they both exhibited the same problem.  Sometimes they would be 90% good…other times they’d fail 50% of the time.  The most interesting thing about this is our PROD system did not exhibit the same issue, even though it is built the same as QA (yeah, right…we know what that means). 

If you look at the link on the picture you’ll see it is generated by a call to the ImageHipChallenge.ashx page and is implemented as shown here. 

From what I can tell this is code that’s been around a while, probably since ASP.NET 1.0 or so.

So, what’s different between PROD and QA?  After stumbling around a bit, I found the Request Management service was turned off in PROD!  If you look under "Services on Server" you’ll find the Request Management service.  Here it is stopped in the SharePoint PROD instance…

…and you can see here it was started on the QA SharePoint instance.

Well, that was interesting, so we turned it off in QA and found something even more interesting.  Our WFE1 box was working 100% (serving up the CAPTCHA images all the time), while the WFE2 box was working 50%.  We knocked heads on this for a while and one of our brilliant guys suggested we try configuring the Request Management Service using PowerShell.  Since we are using two WFEs in a "not very heavily loaded environment" we decided that Request Management probably wasn’t buying us a lot anyway.  So, using Set-SPRequestManagementSettings we disabled Routing and Throttling.

Retrieving the settings with Get-SPRequestManagementSettings provides the following confirmation.

After doing this, our CAPTCHA images are now being served up correctly 100% of the time!  We’re thinking since the Request Management Service was never enabled in production that it is really "not running" while in QA, even when we stopped it in SCA, something about it was still lingering.  Note we did not reboot the QA WFEs during any of this configuration change.

We did have the thought about configuring the Request Management Service to not attempt to route the ImageHipChallenge.ashx requests…and we may still try that.

Oh, the annoying errors from SharePoint!  At least this one didn’t say "please contact your help desk."

This error was experienced on a SharePoint site used for tracking project management tasks. 

There is a task list with a number of Lookup columns that was working perfectly fine for months.  Then, out of the blue, folks started experiencing this error, "The server was unable to save the form at this time.  Please try again."

A search of the web indicates that we are not alone experiencing this problem.  Some of the solutions point to a low memory issue, but our DEV server has 32GB of memory assigned to it, with only about 6-8GB being used, so it’s doubtful that’s the issue!

Some of the potential solutions:

1. Restart the "SharePoint Search Hosting Controller" service.
2. The Free Download Manager Explorer add-on causes an issue.
3. The Security Token Service needs to be recycled or has some other issue.
4. An IISRESET fixes the problem.
5. Enabling content types and adding "link to a document" content type fixes the issue for document libraries.

Until I got to the last potential solution I was having no luck.  Since this was not a document library, I couldn’t add the "link to a document" content type, but I certainly could try adding other content types.

I enabled content types on the list under Advanced Settings.

In the Content Types section, I went to add from existing content types.

From here I simply added the Item content type…one of the simplest system content types.

Voilà!  The edit can now be made to the item and saved successfully!

Now, I didn’t try every item in our list, but the one I was testing on had a file attachment.  Not sure if that has anything to do with the source of the issue, but I did capture a few SharePoint logs along the way.  Here are a couple of things I saw, but couldn’t find anything on the web that helped me much.  These messages seem to point to the RESX files in the C:\Program Files\Common Files\microsoft shared\Web Server Extensions\15\Resources folder and, as I looked inside core.resx and core.en-US.resx, I could not find any string "restricted" within the resource files.

This was seen right after the EditForm.aspx link to the item on which I was experimenting.

pm/Lists/Project%20Tracking%20Tasks/EditForm.aspx?ID=171

There was also a "UserAgent not available, file operations may not be optimized." following it, but I’ve learned to ignore these.