@Nummer378 thanks for your efforts to debug this issue.
To confirm that sudo apt update && sudo apt upgrade work without issue and upgrade other packages.
I thought I would check again that wget also works but got this error before a second attempt succeeded:
$ wget https://apt.syncthing.net/dists/syncthing/candidate/binary-armhf/syncthing_1.5.0~rc.2_armhf.deb –2020-05-01 08:22:44-- https://apt.syncthing.net/dists/syncthing/candidate/binary-armhf/syncthing_1.5.0~rc.2_armhf.deb Resolving apt.syncthing.net (apt.syncthing.net)… 2a03:b0c0:0:1010::99:8001, 82.196.13.137 Connecting to apt.syncthing.net (apt.syncthing.net)|2a03:b0c0:0:1010::99:8001|:443… connected. HTTP request sent, awaiting response… 200 OK Length: 9024674 (8.6M) [application/x-debian-package] Saving to: ‘syncthing_1.5.0~rc.2_armhf.deb’
syncthing_1.5.0~rc.2_armhf.de 68%[=================================> ] 5.86M 531KB/s in 11s
2020-05-01 08:22:56 (537 KB/s) - Read error at byte 6144000/9024674 (A TLS packet with unexpected length was received.). Retrying.
–2020-05-01 08:22:57-- (try: 2) https://apt.syncthing.net/dists/syncthing/candidate/binary-armhf/syncthing_1.5.0~rc.2_armhf.deb Connecting to apt.syncthing.net (apt.syncthing.net)|2a03:b0c0:0:1010::99:8001|:443… connected. HTTP request sent, awaiting response… 206 Partial Content Length: 9024674 (8.6M), 2880674 (2.7M) remaining [application/x-debian-package] Saving to: ‘syncthing_1.5.0~rc.2_armhf.deb’
syncthing_1.5.0~rc.2_armhf.de 100%[++++++++++++++++++++++++++++++++++===============>] 8.61M 434KB/s in 6.5s
2020-05-01 08:23:05 (434 KB/s) - ‘syncthing_1.5.0~rc.2_armhf.deb’ saved [9024674/9024674]
I do have a proxy on Firefox on my laptop, which accesses the PI over ssh and rdp. Turning off the proxy made no difference, the GnuTLS error persisted.
The PI is connected to my router by ethernet and there is no proxy on the router.
It looks like I need to upgrade to Buster, but it’s curious that only ST 1.5.0~rc2 update fails with this error.