Adjustment to the previous post.
Today I switched from a mail that I opened last in a specific folder (where only mail from one source enters) and went back to the INBOX but it showed only the content of the previous folder (with the specific sender). Clicked the SENT folder and it showed the same mails but with the columns of that folder (To instead of From field). Back to INBOX gave me the previous view with the content of the specific folder.
Only after more than 30 seconds the content of the INBOX came in view.
Had just installed the last build: https://runbox.com/app/changelog?since=1594888673