I was getting quite desperate with the issue, since it has remained open for so long.
Because of that, I decided to try everything I could on my own, despite the very little to no chance of success.
I started disbanding my parties and caravans, selling workshops, liquidating stashes in the fiefs, all of the inventory and gear and ultimately kicking compations out of the clan.
Nothing helped, the save games still became corrupted after a couple of days had elapsed in the game.
I tested all of the more recent versions available (1.6.0 and 1.6.1 b272033) and the corruption occurred regardless of the version.
I also tried both creating a new save games with these newer versions and also auto-saves, all of them failed in the same exact manner.
Yesterday evening, while once more using the newest available build (1.6.1.272033) I ended up loading the last working save game that I had.
I opened up the clan-menu and unassigned (chose: "none") the governor of my two fiefs (Diathma & Myzea). Then I opened the inventory and clicked the option that removes all of the inventory at once. After doing that, I pressed cancel the transaction (to get the inventory back) and pressed ok. After that I made a new save game. For whatever reason, the issue is now gone. Also, I noticed that the new .sav files are around half the size of the old ones? Around 297MB each previously, and 149MB each after the change.
So the issue is somehow related to either or to both, the governor(s) of the fiefs (Diathma and Myzea in this case) or some item(s) in the inventory. Whatever it is, this issue is absolutely game breaking and can cause people to loose progress that they've spent hundreds of hours on.
I've uploaded MANY save games with the issue present and today I just uploaded a save game (15/8/2021 16:22 UTC +2) which has the issue fixed.
The save games and the basically pinpointed issue(s) should be more than enough for the devs to find the actual cause behind this issue and more importantly, to fix it.