unknown user name / password combination !!!???

Hi,

I use a SAP DB version 7.3.0.29 with two instances since 01.2003 - so far
without problems.
Today I wanted to make a complete backup with the Database Manager GUI - and
I get the following error message (dbm.prt)

0x00000538 ERR     -24988 DBM      -4008,Unknown user name/password
combination

Under warranty I didn't make any changes with the DBM- or the DBA-
Authorization - and I didn't touch the rest of the DB-configuration.
Using the DataBase Manager you don't have to type in any user/password
combiniation, as the application uses the local system account - which
hasn't changed too.
On 30.07.2003 I made the last complete backup - with no problems.
I made a complete backup with the other instance without any problems too.

Using 'Check - Database' I get the same message and very strange 'Instance -
View Details'
Data, Log, Sessions, Converter Cache Hit Rate, Data Cache Hit Rate are 0%
Connect Possible is 'No' !!!
Database Full is 'No'

Despite all this the application using the DB-istance is running without
problems.

So I'm not very happy with the situation.
Please help.

Thanks and best regards
Georg