Hi,
best guess: your system compiler and standard library implementation were updated together with the system – to versions that are not compatible with your old ROOT installation.
Installing a ROOT version that’s compatible with your new Ubuntu version (i.e. with your new compiler version) should resolve the issue.
You can check https://root.cern/install for a list of available installation methods, e.g. on Ubuntu you can easily install ROOT with one conda command.
The conda package is not directly maintained by the ROOT team, although we collaborate with the maintainers when something breaks. We heard very good feedback though. Ultimately what’s best depends on your situation, but if the conda package works for you it’s fast and painless.