When you cannot connect remotely on DB2 DATABASE due the error below:

Password validation for user  failed with rc = -2146500504

One possible reason for that on Linux and Unix, is: 

The problem is because the the encryption mode used by passwd to store password in /etc/shadow.
The system configuration required sha512 algorithm to store encrypted password in /etc/shadow, but the length of the stored
password (about 100 chars) is too long for DB2. So, to fix this problem you have to change the algoritm to sha256 (and changing the password to the user connecting to DB2), after that the problem will disappear.

Fedora:

vi /etc/pam.d/system-auth

and modify the following line:

# here are the per-package modules (the "Primary" block)
password        [success=1 default=ignore]      pam_unix.so obscure sha512

Edit this line changing the word sha512 to sha256

Ubuntu:

vi /etc/pam.d/common-password

and modify the following line:

# here are the per-package modules (the "Primary" block)
password        [success=1 default=ignore]      pam_unix.so obscure sha512

Edit this line changing the word sha512 to sha256