After way too many hours of frustration I found the culprit: Turning off real-time protection on Windows Security fixes the issue:
> install.packages("gapminder")
Installing package into ‘C:/Users/mario/Documents/R/win-library/4.0’
(as ‘lib’ is unspecified)
trying URL 'https://cran.rstudio.com/bin/windows/contrib/4.0/gapminder_0.3.0.zip'
Content type 'application/zip' length 2031853 bytes (1.9 MB)
downloaded 1.9 MB
package ‘gapminder’ successfully unpacked and MD5 sums checked
The downloaded binary packages are in
C:\Users\mario\AppData\Local\Temp\RtmpGi9Jo9\downloaded_packages
That explains why the problem appeared from one day to the next (Windows 10 update) and why it doesn't show on my work laptop (IT-managed updates).