The issue has NOT been resolved. It's been with Juniper support for 2 weeks.
We've been trying to pinpoint when the issue started, we think it was in the Summer, when Juniper advised a firmware upgrade for a different reason. We come from 12.1X44-series, which is no longer supported, but for which the config worked.
Right now, the config has changed, but mostly typical policies were added and some stuff has been tried to block a video stream (all possible ways failed, Juniper had no conclusive answer on how to block RTMP streams without specifying the IP of the website).
Anyhow, after removing nearly all changes, I still can't get the VPN to stay up in the 12.3X48-series (upgraded to the latest version this noon). So it might be a firmware issue. That's my main suspect for now. It's 100% definitely a firewall issue, not a network issue.
Has anyone a working config for IPSec VPN with ShrewSoft, which works on 12.3X48 series?
If so, please be so kind to share it here or privately.
If I get something to work, I'll post it here again.
Oh, also, the question "are dynamic VPN licenses required?". The first support engineer claimed it was necessary, the last guy told me they are not required for the config as suggested in this topic...