Is this the only page you get 400?

Yes, the server replies with HTTP/400.
Is there any test I can perform to help debug this issue?

Please activate debug mode in /application/config/config.php and see if you get a more elaborate error.

I'm getting a full-page PHP warning after enabling debug mode (attached).

Please upgrade the latest LimeSurvey build released today and let us know if that fixed your issue.

:-)

I've upgraded to 2.50+ build 160512, set ug+rw permissions for apache on tmp, upload and application/config, but I'm still getting a 400 error.
Would a wire level protocol capture help?

Did you set a custom template as standard template in global settings?
If yes, can you set it to 'default' and try again?

I do have a custom template, but it is not set as default.
The default template is 'default' (Flat and Modern).
The survey I'm testing on is also using the 'default' theme.

I can see from the debug output that the clear_logo template is trying to be used.
Clear_logo is not a factory template, anymore and not supported.
Can you check again, please?

The only files I copied over from the old install were config.php and a template.
I grepped for 'clear_logo' and did not find it.
This indicates that the 'clear_logo' reference comes from the database, that was in fact upgraded from an earlier version.
I dumped the database and sure enough, there are several references to 'clear_logo' on the followin tables: 'lime_permssions', 'lime_surveys' and 'lime_templates'.
I'll try to replace those references with 'default' and will report back.

Having fixed the 'clear_logo' issue, I enbled the debug mode ang got a stacktrace (attached) to go with the 400 error.
Hopefully you will be able to figure out what is wrong.

Ok, thank you - that helped to locate the error - we will provide a patch soon.

Thank you for your prompt response! :-)

Fix committed to master branch: http://bugs.limesurvey.org/plugin.php?page=Source/view&id=18997

Can you check the patch and please report if the issue is fixed for you?

I applied the patch, but the issue remains.
I was going to upload the stacktrace, but it is virtually the same as before.

Fix committed to master branch: http://bugs.limesurvey.org/plugin.php?page=Source/view&id=19005

I'm still getting the attached stacktrace after applying both patches.

Try to upgrade your PHP version to at least 5.4.

The issue in the stacktrace is a new one.
Probably you did an error when patching?
You should wait for the next release version and do a full update then to make sure that all file are genuine.

Version 2.50+ Build 160516 released

Hello,

I installed latest build 160517 with a clean database but I can still trigger this issue.
Do you consider that upgrading to php 5.4 is the only workaround?
Thank you in advance.

If you look into your file /application/config/config.php,
what is set in the line 'Urlformat' ?
Did you do any custom settings in config.php ?

So, after reading in detail the error logs in the last zip file, it became clear.

Here the requested URL reported by CHttpException:

CWebApplication->runController("admin/questions/sa/importview")

It should be something like:

CWebApplication->runController("admin/questions/sa/importview/surveyid/{A_SURVEY_ID}")

IE: there is no surveyid in the URL.

So, you can reproduce the bug by accessing directly to the url :
admin/questions/sa/importview

But the button "import a question" itself contains a link pointing to admin/questions/sa/importview/surveyid/{A_SURVEY_ID}. So even if the surveyid would have been wrong for any reason, the CHttpException should have shown :
CWebApplication->runController("admin/questions/sa/importview/surveyid/{something}")

So at least the latest CHttpException ziped file can't have been generated clicking on the button. I bet it has been generated just reloading a page with a wrong URL (I can be wrong).

Here the code to generate the button itself :
https://github.com/LimeSurvey/LimeSurvey/blob/master/application/views/admin/survey/surveybar_view.php#l63-l66

There's no way this code is going to generate a url such as the one indicated in the last zip file.

So, just update to last version, and everything should work like a charm.

Version 2.50 Build 160613 released