Module upload problem. What could be the cause?

[WMD]

CMS 1.7.1. Tried to install a FormBuilder module (latest version):
- downloaded XML file from the site (FormBuilder-0.6.2.xml);
- started to upload it in the Modules section;
- shortly after received and error message saying:
Error parsing XML file. Please make sure you are uploading a .xml file and not a .tar.gz or zip file

- In the debugging mode I can see a lot of stuff reletated to MySQL, starting with:

Debug: (0.032282) - (2459480)

(mysql): SET NAMES 'utf8'
Error (0):
Debug: (0.032898) - (2474772)

(mysql): SELECT sitepref_name, sitepref_value from amp_siteprefs
Error (0):
Debug: (0.037495) - (2562332)

(mysql): SELECT * FROM amp_userplugins
Error (0):

Tried also the previous version of the Form Builder - no luck. Can anyone please advise where the root cause of this problem is?