I've done a fresh install (I started with 6.1.4 and updated to 6.1.7) on a Debian VM connecting to my company's central MS SQL Server with FreeTDS. When I try to log into Sugar I get a blank page. In Firebug I can see that the server is returning a 500 error, but the Apache logs are empty and sugarcrm.log shows no errors as well.

After getting the blank error page, I can navigate manually to my server root and use Sugar as normal, without additional errors.

Here's a trace from sugarcrm.log of one of the failed login attempts:

Code:
root@sucrose:/var/sugarcrm# cat login-trace.log | grep -vi 'query execution time' | sed -e 's/Mon Nov 14 07:29:17 2011 .4833..d67297fe-91e8-efd6-551c-438fb4bf4a32..INFO.//g' | sed -e 's/Mon Nov 14 07:29:17 2011 .4833..-none-..INFO.//g'
 Found language file: en_us.lang.php
 Found extended language file: en_us.lang.ext.php
 using FreeTDSManager DBManager backend
 using FreeTDSHelper DBHelper backend
 Connect:Resource id #87
 Query:SELECT id, name, symbol, conversion_rate FROM currencies WHERE status = N'Active' and deleted = 0
 Query:SELECT category, name, value FROM config
 Query:SELECT id FROM outbound_email WHERE type = N'system'
 Query:SELECT * FROM outbound_email WHERE id = N'11645415-7844-a55c-8107-482290cceb02'
 Found language file: en_us.lang.php
 Found extended language file: en_us.lang.ext.php
 Found language file: en_us.lang.php
 Found extended language file: en_us.lang.ext.php
 Query:SELECT count(*) as the_count FROM config WHERE category=N'info' AND name=N'sugar_version' AND CAST(value AS varchar(8000)) = N'6.1.7'
 Query:SELECT category, name, value FROM config
 Query:SELECT id FROM outbound_email WHERE type = N'system'
 Query:SELECT * FROM outbound_email WHERE id = N'11645415-7844-a55c-8107-482290cceb02'
 Query:INSERT INTO tracker (monitor_id,module_name,date_modified,action,session_id ) VALUES ( N'39a59dd5-060e-2493-3d6b-4ec1175ac6fd',N'Users',N'2011-11-14 13:29:17',N'authenticate',N'pit4jioca24l5n7vm7stiuapb1')
 Query:SELECT  TOP 1 users.* FROM users  WHERE user_name = N'kevin' AND deleted=0
 Query:SELECT u1.first_name, u1.last_name from users  u1, users  u2 where u1.id = u2.reports_to_id AND u2.id = N'd67297fe-91e8-efd6-551c-438fb4bf4a32' and u1.deleted=0
 Query:SELECT  TOP 1 users.* FROM users  WHERE users.id = N'd67297fe-91e8-efd6-551c-438fb4bf4a32'  AND users.deleted=0
 Query:SELECT u1.first_name, u1.last_name from users  u1, users  u2 where u1.id = u2.reports_to_id AND u2.id = N'd67297fe-91e8-efd6-551c-438fb4bf4a32' and u1.deleted=0
 Query:SELECT ea.*, ear.* FROM email_addresses ea
 Query:SELECT contents FROM user_preferences WHERE assigned_user_id=N'd67297fe-91e8-efd6-551c-438fb4bf4a32' AND category = N'global' AND deleted = 0
 Query:SELECT  TOP 1 * from users where user_name=N'kevin' AND user_hash=N'0495708c4a7c6f8f15eb00824b2cda5f' AND (portal_only IS NULL OR portal_only !='1') AND (is_group IS NULL OR is_group !='1') AND status !=N'Inactive'
 Query:SELECT  TOP 1 users.* FROM users  WHERE users.id = N'd67297fe-91e8-efd6-551c-438fb4bf4a32'  AND users.deleted=0
 Query:SELECT u1.first_name, u1.last_name from users  u1, users  u2 where u1.id = u2.reports_to_id AND u2.id = N'd67297fe-91e8-efd6-551c-438fb4bf4a32' and u1.deleted=0
 Query:SELECT ea.*, ear.* FROM email_addresses ea
 Query:SELECT  TOP 1 users.* FROM users  WHERE user_name = N'kevin' AND deleted=0
 Query:SELECT u1.first_name, u1.last_name from users  u1, users  u2 where u1.id = u2.reports_to_id AND u2.id = N'd67297fe-91e8-efd6-551c-438fb4bf4a32' and u1.deleted=0
 Query:SELECT  TOP 1 users.* FROM users  WHERE users.id = N'd67297fe-91e8-efd6-551c-438fb4bf4a32'  AND users.deleted=0
 Query:SELECT u1.first_name, u1.last_name from users  u1, users  u2 where u1.id = u2.reports_to_id AND u2.id = N'd67297fe-91e8-efd6-551c-438fb4bf4a32' and u1.deleted=0
 Query:SELECT ea.*, ear.* FROM email_addresses ea
 Query:Select * from  versions  where  name=N'Chart Data Cache'
 Query:Select * from  versions  where  name=N'htaccess'
 Query:Select * from  versions  where  name=N'Rebuild Relationships'
 Query:Select * from  versions  where  name=N'Rebuild Extensions'
 Query:SELECT category, name, value FROM config
 Query:SELECT id FROM outbound_email WHERE type = N'system'
 Query:SELECT * FROM outbound_email WHERE id = N'11645415-7844-a55c-8107-482290cceb02'
 Query:SELECT category, name, value FROM config
 Query:SELECT id FROM outbound_email WHERE type = N'system'
 Query:SELECT * FROM outbound_email WHERE id = N'11645415-7844-a55c-8107-482290cceb02'
 Query:SELECT id, first_name, last_name, user_name from users WHERE status=N'Active' AND is_group=0 AND portal_only=0  ORDER BY user_name ASC
 Query:select count(*) count from users where status=N'Active' and deleted=0 and is_admin='1'
 Query:select count(*) count from users
 Query:SELECT count( DISTINCT users.id ) user_count FROM tracker, users WHERE users.id = tracker.user_id AND  tracker.date_modified >= N'2011-10-14 07:29:17'
 Get One: . |select id from tracker order by date_modified desc|
 Query:select id from tracker order by date_modified desc
I've looked over some of the other "Blank Page After Login" threads and double checked that my PHP memory limit is 128MB, and the sugar dir is owned by the Apache user. Any ideas what the problem could be?