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

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).