Dear @opensam
Thank you for your answer.
I use the latest version of Debian 11 (Bullseye) where versions of Tomcat and Java OpenJDK match those required for AOS 8 (i.e. 9 and 11, respectively). I have not tried more up-to-date versions of these packages.
As for building the application, I followed the workflow described in this official video. I cloned the open-suite-webapp repository in which I called for all submodules, updated them and built the application, which more or less reads like:
$ git clone https://github.com/axelor/open-suite-webapp
$ cd open-suite-webapp
$ git submodule init && git submodule update
$ ./gradlew clean build
After that, I copied the resulting application archive to the proper location for Tomcat, before restarting it:
$ cp build/libs/axelor-erp-8.0.4.war /var/lib/tomcat9/webapps/ROOT.war
$ systemd start tomcat9 && journalctl -f -u tomcat9
Note that prior to building the application, I had cloned the axelor-addons repository into open-suite-webapp/modules, as described here.
In order to investigate the error I encounter and reported in this post, I ended up removing this addons repository from the modules directory, and rebuilt the application. It turned out that the error went away. However, I do need one addon (PrestaShop). Leaving the addons repository solely with the directory of this addon and rebuilding the application turned out successful as well.
Therefore some addon(s) seem to be at the origin of the error stated here.
I have not taken the time to find out which of them cause the issue.