SRX5308 35% packet loss on DSL (solved)

I was called to a customer's site to replace an aged FVL328 VPN router by a new SRX5308. They have a business ISDN access with an Arcor DSL modem and a NTsplit (Arcor has been bought by Vodafone quite a while ago and that stuff ist pretty old).

The FVL328 was used to connect the LAN to the DSL, obviously. After some hassle with the DSL login, which could be solved with the help of Vodafone's business hotline, I did get the SRX5308 online. I could successfully ping Google's public DNS resolver. Unfortunately, my pings showed about 35% packet loss. Surfing the web was virtually impossible. Switching back to the FVL328 for a test - and the problem was gone.

After some playing around with various options of the SRX5308, I found the cause of the problem: The DSL modem (built in 2006) apparently has a 10BaseT-NIC. As long as the link speed in the SRX5308 is set to auto, you get packet loss. As soon as you set it to 10BaseT-FullDuplex, everything is fine.

I hadn't seen this precise problem (networks not working because of wrong link speed settings) for a couple of years, so it took me quite some time to find it. That's why I thought, I'd post it here for others to find.