There are a number of possible causes for login problems. Some of the more common causes are:

Login in 1.3.x are new. Take a look at this screen guide:
http://www.ledgersmb.org/help/install/Ubuntu_Installation_Tutorial_Ledge...

Magic Characters in passwords:
The legacy code from SQL-Ledger has some issues with parsing passwords that contain either the equals sign ("=") or the ampersand (&). This is gradually being resolved in the LedgerSMB codebase but for now avoid using either of these characters in your passwords. The issue is that the equals and ampersand characters are used as delimiters in query strings and posted content - this is being worked on so should not be an issue "soon".

Googlebait: Symptoms of this include "Error! Cannot connect to datasource" and "no database driver specified and DBI_DSN env var not set at LedgerSMB/Form.pm" (suspect this is for admin password).

Password "popup"
Installed in a fresh install of Ubuntu. All seemed fine on in the evening when I went home for the weekend. (I was
able to login) Now when I login I get an other password request popup opening with: A username and password are being requested by http://127.0.0.1. The site says: "LedgerSMB"

The likely issue here is that the initial password creation temporarily valid, with the idea that the individual setting the
password may not be the application user. Consequently the password has expired and you cannot log in.

To solve the Password "popup", you can do one of a couple things:
By: Chris Travers

1) You can locate your pg_hba.conf file and change the authentication
type to "trust" briefly while logging in and changing your password.
This requires telling PostgreSQL to reload its files (or restarting
PostgreSQL)

2) You can log into PostgreSQL with something like: sudo -u postgres psql
and then change the expiration date. Something like:

ALTER USER 'myusername' WITH VALID UNTIL '2011-11-09';

Then when you log in, you should be able to change your password under
the preferences screen or if it will expire within a week, it should
pop up that screen when you log in, with a warning on top.