Can't cross sign new element-linux session from element-android

#### Describe the bug
I can't cross sign my new device. All my contacts who verified me see a red shield next to my name.

#### To Reproduce
Steps to reproduce the behavior:
1. In Element-Android go to 'Settings' -> 'Security & Privacy' -> 'Show All Sessions'
2. Click on the new unverified session
3. Click on 'Manually verify by text
4. The session key is matching, click on "Verify"
5. See the error message above "Error - Cannot sign from this account, public and/or privateKey Unknown"

Extra: verifying by emoji doesn't work as well, but the success screen is shown without any error message, even tho the device doesn't get cross signed

#### Expected behavior
1. Cross signing should work
2. if the emoji-verification fails I expect an error message, not an success screen

#### Screenshots

| step 2 | step 3 | step 4 | extra
| --- | --- | --- | --- |
|  |  |  |  |

#### Smartphone (please complete the following information):
- Device: Nexus 5
- OS: Android 10 (CrDroid ROM)

#### Additional context
- App version and store: 1.1.0 - F-Droid
- Homeserver: not matrix.org, but current Synapse (and problem was first noticed ~half a year ago)