With the help of the support team, I found out that the connection error was not caused by the Mikrotik devices. It is almost certainly a problem with a fibre optic cable. This is probably what caused the timeouts.
I am having massive problems connecting two Mikrotik switches, both flashed with the latest SwOS 2.13, to a LAG and keeping them running. I have tried several different devices together, but nothing has worked.
With the LAGs I tried 2 x 1GB copper and also 2 x SFP+. I tried these connections (tagged) between different switches. I have the following switches in use:
CRS326-24G-2S+
CRS354-48G-4S+2Q+
CRS309-1G-8S+
CRS317-1G-16S+
CRS326-24S-1Q+RM
I have tested these co-crotics alternately. I could not establish a stable connection with any pair. After a few seconds or minutes, the network had timeouts that disappeared after a reboot but came back after a short time.
I noticed that a connection with a Windows Storage Server, which I set up there with 4 x 1 GBit Ethernet as LACP (dynamic), seems to work. However, I still need to observe this more closely.
It took me a lot of time to identify the error in the LAG of the Mikrotik products. Now that I only use individual connections, the network works without any problems, so I think I can also rule out a misconfiguration.