Role could not be found when registering with Firefox

We have all users in LDAP, so we let also register them (= first login) via LDAP. They should be assigned the roles Blogger and Customer (= hard coded in LDAP module). However, this only works with a Chromium browser. Doing so with Firefox (similar language settings), we get

Zeile #1: Rolle: Kunde konnte nicht gefunden werden
(means: Role: Customer could not be found)

Traceback (most recent call last):
File “/home/erp/frappe-bench/apps/frappe/frappe/”, line 61, in application
response = frappe.handler.handle()
File “/home/erp/frappe-bench/apps/frappe/frappe/”, line 21, in handle
data = execute_cmd(cmd)
File “/home/erp/frappe-bench/apps/frappe/frappe/”, line 56, in execute_cmd
return, **frappe.form_dict)
File “/home/erp/frappe-bench/apps/frappe/frappe/”, line 1026, in call
return fn(*args, **newargs)
File “/home/erp/frappe-bench/apps/frappe/frappe/integrations/doctype/ldap_settings/”, line 59, in login
user = authenticate_ldap_user(frappe.as_unicode(args.usr), frappe.as_unicode(args.pwd))
File “/home/erp/frappe-bench/apps/frappe/frappe/integrations/doctype/ldap_settings/”, line 114, in authenticate_ldap_user
return create_user(params)
File “/home/erp/frappe-bench/apps/frappe/frappe/integrations/doctype/ldap_settings/”, line 138, in create_user
user = frappe.get_doc(params).insert(ignore_permissions=True)
File “/home/erp/frappe-bench/apps/frappe/frappe/model/”, line 216, in insert
File “/home/erp/frappe-bench/apps/frappe/frappe/model/”, line 741, in _validate_links
File “/home/erp/frappe-bench/apps/frappe/frappe/”, line 351, in throw
msgprint(msg, raise_exception=exc, title=title, indicator=‘red’)
File “/home/erp/frappe-bench/apps/frappe/frappe/”, line 337, in msgprint
File “/home/erp/frappe-bench/apps/frappe/frappe/”, line 310, in _raise_exception
raise raise_exception(msg)
LinkValidationError: Zeile #1: Rolle: Kunde konnte nicht gefunden werden

Any idea what Firefox and Chromium do differently?