I am having the same error and I think I can explain what it means and provide a short-term fix.
In the .../mod/bigbluebutton/lib.php file on line 244 there is a call to the database using
"$DB->record_exists('bigbluebutton', 'grade', -$scaleid)", however this seems to be a pre-2.0 function, according to this page: http://docs.moodle.org/en/Development:DML_functions_-_pre_2.0#Seeing_if_any_records_exist_match_a_given_criteria
Specifically it looks like they are attempting to query the 'bigbluebutton' table and find the field titled 'grade' where value equals the variable $scaleid.
However, in 2.0+ the database query should be formated like this:
Furthermore, on my installation of Moodle (2.0.2 Build: 20110221), the BigBlueButton (v. 2010123100) database table does not have a field titled 'grade'. So there seems to be several issues with this line of code and, frankly, I'm not entirely sure what BigBlueButton is trying to do here, so I cannot say what the proper solution should be.
Judging from the comments for that block of code:
"Checks if scale is being used by any instance of bigbluebutton. * This function was added in 1.9 * This is used to find out if scale used anywhere"
It seems that the database is queried by BigBlueButton in order to find any custom grade scales that have been applied, however since this line of code seems to be either inaccurate or out of date, it causes the error we observed.
I was able to solve the problem in the short term by commenting out that line and the lines surrounding it (an If statement) and simply have that function always return false. This quick fix solved the error you and I were getting and I have not observed any further errors or complications in either Moodle or BigBlueButton as a result of my change, though I am not doing anything related to grades in BigBlueButton so there could be problems that are not apparent from my testing.
In the long term it would probably be best to alert the maintainer of the BigBlueButton module and tell them that they have a likely bug in their code. The author of those php files is listed as Fred Dixon (ffdixon [at] blindsidenetworks [dt] org), so perhaps an email to him would be appropriate? I think that is the approach I will be taking.
I hope that helps!
My background: though this is my first time posting on the forum, I've been reading the forums for about a year and am the lead programmer for my college's moodle installation.