FortiOS 7.6.0 Build3401 - RadSec TLS 1.3 not possible

[fabs]

Hello all,

I have a problem, probably in connection with my Fortigate100F to establish a RadSec TLS 1.3 connection from a UniFi AC Pro AP to an external RadSec Server (RADIUS). The connection is made via a Ubiquiti UniFi AC Pro AP and Ubiquiti UniFi Application 8.5.6 and RadSec is official supported.
The connection via RADIUS works, but as soon as I activate TLS for this connection with the required certificates (root, client, private key), no connection is established.

In the log files I do not actually see that the connection is blocked. The only thing I see is action=“close” - What does this mean?
When I add the external IP in my exclusion list from my SSL Deep Inspection, I can't connect either.
Even if I temporarily deactivate IPS, SSL and App Control, I cannot establish a connection.
I can establish a telnet connection to the RadSec server from the Fortigate and from the UniFi AC Pro AP. However, this is terminated immediately

date=2024-10-21 time=13:42:13 eventtime=1729510933581869890 tz="+0200" logid="0000000013" type="traffic" subtype="forward" level="notice" vd="root" srcip=192.168.10.237 srcport=58594 srcintf="VLAN10" srcintfrole="lan" dstip=206.xxx.xxx.xxx dstport=2083 dstintf="wan1" dstintfrole="wan" srccountry="Reserved" dstinetsvc="DigitalOcean-DigitalOcean.Platform" dstcountry="Netherlands" dstregion="North Holland" dstcity="Amsterdam" dstreputation=4 sessionid=16267522 proto=6 action="close" policyid=2 policytype="policy" poluuid="b08ab9d0-cf8b-51ec-7409-582ae59457cf" policyname="VLAN10 -> WAN" service="DigitalOcean-DigitalOcean.Platform" trandisp="snat" transip=xx.xx.xx.xx transport=58594 appid=47013 app="SSL_TLSv1.3" appcat="Network.Service" apprisk="medium" applist="block-high-risk" appact="detected" duration=2 sentbyte=1015 rcvdbyte=4293 sentpkt=11 rcvdpkt=8 sslaction="exempt-addr" utmaction="allow" countapp=2 countssl=1 srchwvendor="Ubiquiti" mastersrcmac="d0:xx:xx:xx:xx:xx" srcmac="d0:xx:xx:xx:xx:xx" srcserver=0 utmref=16267522:1729510934

What else could cause a problem here?

Thanks
fabs