This is caused by a bug between wxWidgets, and the new Visual Studio 2010 compiler that was used.

A fix for it is available, in the form of a hotfix, while we wait for a new BOINC to be released.
Rom Walton wrote:

For anybody running into the 400 error on any project, can you give this private drop of BOINC Manager a try?

x64: http://boinc.berkeley.edu/dl/boincmgr.180214.x64.zip
x86: http://boinc.berkeley.edu/dl/boincmgr.180214.x86.zip

To use the drop, download the correct version of BOINC Manager. You can see if you have a 32bit (x86) or 64bit (x64) version by opening BOINC Manager->Help->About BOINC Manager. There it will state if it's a 32bit or 64bit version.

When you know this, download the above compressed file.
Next exit BOINC completely: File->Exit, Check Stop running tasks when exiting Manager?, OK. If you do not see this dialog, and you want to, open BOINC Manager again, then go Tools->Options and check "Enable Manager exit dialog?"->OK, then do File->Exit.
Next open the compressed file, Organize->Select all, Organize->Copy.
Now navigate to your BOINC Programs directory.

Default BOINC Program directories:
Windows 32bit: C:\PROGRAM FILES\BOINC
Windows 64bit: C:\PROGRAM FILES (x86)\BOINC for 32bit BOINC, C:\PROGRAM FILES\BOINC for 64bit BOINC

In there do Organize->Paste.

Next restart BOINC Manager.
Start->All Programs->BOINC->BOINC Manager.

Now you can use the Add Project wizard to add the project of choice:
Add Project->Add project or account manager->Add project button in Simple view.


Or Tools->Add project or account manager->Add project in Advanced view.




Where you can now choose the project of choice as before.

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A new version of BOINC (7.2.42) is being tested, for Windows only at this moment. Linux and Macintosh versions are to follow, but the problem here is that the developers have already switched over to wxWidgets 3.0 on their machines, so they'll need to make a new virtual machine image with wxWidgets 2.8, to be able to build 7.2 versions.

They also thought the problem was only happening on BOINC 7.3, but finally the message got through that it's the 7.2 recommended that has the problems. With thanks to all that reported their problems here.

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