Could you try resetting the user in the 501-group to a super simple password (to rule out problems related to character sets)? Also - what is the exact error message that is shown when you try to log-in?
If it is unable to validate the password, that can sometimes also mean that there was a crash in the openldap-container, so in that case you can try the following command and have a look at the uptime column (i.e. if the openldap container crashes, it restarts automatically - in which case it will have a significantly shorter uptime than the other containers);
Out of curiousity - how big is the machine/vm that you are running the sync endpoint on? I have sometimes run into problems with the openldap container crashing mysteriously when I try to run the sync endpoint on (virtual) hardware with too little RAM etc (I personally think that 2gb ram is the minimum for openldap to run, although I don’t know the official requirements for that)