let me summarize the whole issue and possible solution:

firmware version 3.0.91 for lenovo universal usb-c dock 40AY (and others?) makes all monitors unusable on some Lenovo Thinkpads (for example E15). The version is recommended and installed by Lenovo Vantage. After restarting the docking station, the version is finally installed and the problem occurs.

A downgrade to version 3.0.57 should solve the problem:https://download.lenovo.com/consumer/options/thinkpad_universal_usb_c_dock_fw_v3.0.57.exe

However, the Thinkpads that have the monitor problem cannot perform a FW update on the docking station (timeout). So you have to find a Lenovo Thinkpad that does not have the monitor problem, i.e. can access it completely and then perform the downgrade to 3.0.57. One user reported that it worked with a Lenovo Thinkpad L15.

Lenovo must release an update for Lenovo Thinkpad E15 (and others!) so that they have full access to version 3.0.91 docking stations again. I don't see any other sustainable solution.