Squid/SquidGuard Ldap User Cannot Authenticate if Password has special character

[w.hackl]

Hello ,
I'M using

PFSense 2.4.5_p1
Squid 0.4.44_31
squidGuard 1.16.18_7

with a LDAP Auth Option for all Users to AD which is working fine except following Problem :

if the user enters a AD Password with german umlaut ( in this case ü [ u umlaut ] ) no authentication is possible to squid / squid guard.

related config in squid :

Spoiler

auth_param basic program /usr/local/libexec/squid/basic_ldap_auth -v 2 -b 'dc=ad-domain,dc=somewhere' -D 'cn=administrator,cn=users,dc=ad-domain,dc=somewhere' -w 'workingpassword' -f 'samAccountname=%s' -u 'uid' -P -H 'ldap://X.X.X.X:3268'
auth_param basic children 5
auth_param basic realm Proxy Anmeldung
auth_param basic credentialsttl 5 minutes
acl password proxy_auth REQUIRED
authenticate_ip_ttl 5 minute

There are no errors in squid / squidguard logs , except the TCP_DENIED lines because of the "wrong" password.

If the user changes the password with ü to u everything works fine.

Any suggestions or commands to force squid to use the "right" characterset ?