SSL certificate problem: unable to get local issuer certificate

My app that scrapes a small amount of data has been working fine for quite a while as deployed on shinyapps.io, but a few days ago it stopped working with this error from io:

Error in value[3L] :
SSL certificate problem: unable to get local issuer certificate
Calls: local ... tryCatch -> tryCatchList -> tryCatchOne ->
Execution halted

I tried redeploying (it works fine locally), but get the same. Any ideas?

Thanks,
Dave