Element iOS 1.0.2 reports error when trying to disconnect from identity server

Several clients of the first hour of my private matrix-synapse homeserver are unable to disconnect from a formerly given identity server url.

In the first period of riot it was necessary to fill out a identity server url to register or login to a matrix-server.

In the meanwhile this function is deprecated because nobody needs an identity server to use the matrix ID to start direct chats or participate to group chats.

Some of my friends thatwhy still have an old identity server adress saved in their profiles. Now with switch to element iOS and switch to cross signing they want to get rid of this.
When disconnecting the element iOS App just reports "Fehler" ("Error") - nothing else, no hint.

We tried to clear the caches (element and system) no diff.
We switched over from german language to english -> no diff.
(there was a little hope to try this because verifying other iOS clients is not possible when the app runs under german language)

Cross Signing is enabled on these clients and we can verify each other - but we can't get rid of those identity server adresses.

What settings in client / matrix-synapse could cause such errors?
What Workaround might exist to get these urls deleted?

The matrix-synapse is running under version 1.18.0
The iOS Clients run 1.0.2
All iPhones have iOS 13.6 installed.

My server settings concerning identity servers are the following:
```

# The identity server which we suggest that clients should use when users log
# in on this server.
#
# (By default, no suggestion is made, so it is left up to the client.
# This setting is ignored unless public_baseurl is also set.)
#
#default_identity_server: https://matrix.org

# The list of identity servers trusted to verify third party
# identifiers by this server.
#
# Also defines the ID server which will be called when an account is
# deactivated (one will be picked arbitrarily).
#
# Note: This option is deprecated. Since v0.99.4, Synapse has tracked which identity
# server a 3PID has been bound to. For 3PIDs bound before then, Synapse runs a
# background migration script, informing itself that the identity server all of its
# 3PIDs have been bound to is likely one of the below.
#
# As of Synapse v1.4.0, all other functionality of this option has been deprecated, and
# it is now solely used for the purposes of the background migration script, and can be
# removed once it has run.
#trusted_third_party_id_servers:
# - matrix.org
# - vector.im
```

So everything should be outcommented.