With the help of @IpWizard I managed to solve the following import problem.

I tried to import a large batch of transactions of an old account.
It was a .csv file and I used software (iCreateOFX) to make it into an .ofx file. This normally works.

However every time I would import the transactions it would show the number of transactions imported, but it showed 0 (zero) payees imported.
After that, when I would click on the account to see the transactions in the ledger it would "hang" and the ledger never showed.

No matter what I tried nothing seemed to work (importing as .csv, importing as .ofx, adding the transactions to an old file that worked before).

Then I asked @IpWizard for support. And he checked what version I was running. Turned out I was running a Nightly version (master branch) instead of a stable version. See: https://kmymoney.org/download.html
The installation of the stable branch. The problem was solved!