It seems that I had the same problem. Most shared folders where not accessible anymore and password rejected. Tried to reboot it and this had no effect.
I resolved it by dis-enabling the windows file service / save / enabling the windows file service again. Now the access to the shared folders is normal again.
Called this one to early, still have problems with accessing the shares, it looks like the groups are not functioning anymore for windows sharing.
I have a user that belongs to a group and the login is rejected for that user. If I give the user access through the user profile and not group profile access is granted.
Synology must have busted something again.