Unreadable characters in ggplot graphs - running R in Jupyter on Linux

I have been running R in Jupyter for some time, but from one day (and possibly kernel update) to another things have changed for the ggplot fonts, where both the legend and axis labels are garbled... For example, doing a histogram gives me the following graph:

code:

ggplot(NVemo, aes(x = CognitiveLoad)) +
    geom_histogram(fill = "lightblue", color = "white") +
theme_classic()

Specs for my system:

Linux mint kernel 5.4.0-64-generic

and R version code produces:

platform       x86_64-conda_cos6-linux-gnu 
arch           x86_64                      
os             linux-gnu                   
system         x86_64, linux-gnu           
status                                     
major          3                           
minor          6.1                         
year           2019                        
month          07                          
day            05                          
svn rev        76782                       
language       R                           
version.string R version 3.6.1 (2019-07-05)
nickname       Action of the Toes