HCL created an entry in the knowledge base with an important hint.

When deleting calendar entries from the view “Weekly Planner” with the DEL function on the keyboard multiple entries in succession are removed !!

https://support.hcl-software.com/csm?id=kb_article&sysparm_article=KB0128302

There´s a workaround for this issue which is mentioned in the above link:

> When deleting calendar entries, do not delete them in the Weekly Planner view.
> When deleting a calendar entry, do not use the DEL key.

And of course HCL is working on a fix.