Could you exclude the Dynamic Cupshare plugin?
I set it the way that only clan members share cupboards.
Problem is that players can not loot unlocked cupboards in raidable bases now.
They have to destroy the cupboard but the stash then often is hidden in the ground and
can not be looted anymore.

It is excluded. You cannot loot a cupboard unless you authorize on it first. There's absolutely no reason you shouldn't be able to authorize on these cupboards that have no owner, and no players authorized on it. And there is certainly no reason you can't loot a cupboard that you are authorized on. This doesn't make any sense :p

Update Dynamic Cupshare

2.7.7 is the newest version and that's the one I got.

explain what happens. what message do you get?

No message. 
It seems the problem that this player told me about doesn't have to do with Dynamic Cupshare.
We tried it again and he could access the tc.
It seems sometimes tc's and chests can not be accessed maybe because they're in the ground a bit.

yea, I don't think it's Dynamic Cupshare either

so is the issue when looting, or when authorizing? you said it affects chests too, so it is only for looting then?

so then you would look for CanLootEntity, CanUseLockedEntity, OnLootEntity in your plugins

can post those plugin names here and I can see if they're the issue or not

The player mentioned that when trying to loot a tc or chest sometimes he won't get access.
No message or anything. He just couldn't loot.
I have no plugins which forbid to get access for any chests except Dynamic Cupshare to only allow clan members to open a tc.

No clue then