pdfsam fails to run a merge on Ubuntu 20.04 with error

After installing pdfsam, trying to merge two or more pdf files will fail with the following error:

ERROR [18:52:18]: Unexpected error
java.lang.NoClassDefFoundError: javax/validation/ParameterNameProvider
at org.hibernate.validator.HibernateValidator.createGenericConfiguration(HibernateValidator.java:33)
at javax.validation.Validation$GenericBootstrapImpl.configure(Validation.java:173)
at org.sejda.core.validation.DefaultValidationContext.<init>(DefaultValidationContext.java:41)
at org.sejda.core.validation.DefaultValidationContext.<init>(DefaultValidationContext.java:36)
at org.sejda.core.validation.DefaultValidationContext$DefaultValidationContextHolder.<clinit>(DefaultValidationContext.java:70)
at org.sejda.core.validation.DefaultValidationContext.getContext(DefaultValidationContext.java:50)
at org.sejda.core.service.DefaultTaskExecutionService.validate(DefaultTaskExecutionService.java:104)
at org.sejda.core.service.DefaultTaskExecutionService.validateIfRequired(DefaultTaskExecutionService.java:97)
at org.sejda.core.service.DefaultTaskExecutionService.execute(DefaultTaskExecutionService.java:66)
at org.sejda.core.service.DefaultTaskExecutionService.execute(DefaultTaskExecutionService.java:58)
at org.pdfsam.task.TaskExecutionController.lambda$request$0(TaskExecutionController.java:87)
at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
at java.base/java.lang.Thread.run(Thread.java:834)
Caused by: java.lang.ClassNotFoundException: javax.validation.ParameterNameProvider
at java.base/jdk.internal.loader.BuiltinClassLoader.loadClass(BuiltinClassLoader.java:581)
at java.base/jdk.internal.loader.ClassLoaders$AppClassLoader.loadClass(ClassLoaders.java:178)
at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:522)
... 14 common frames omitted

1)
lsb_release -rd
Description: Ubuntu 20.04.1 LTS
Release: 20.04

2)
apt-cache policy pdfsam
pdfsam:
  Installed: 4.0.4-1
  Candidate: 4.0.4-1
  Version table:
 *** 4.0.4-1 500
        500 http://mirror.ratiokontakt.de/mirror/ubuntu focal/universe amd64 Packages
        500 http://mirror.ratiokontakt.de/mirror/ubuntu focal/universe i386 Packages
        100 /var/lib/dpkg/status

3) What you expected to happen: The pdf files to be merge in a new one.

4) No output but instead the program has a red exclamation icon that on click reveals the above error has happened and that appears to be the reason no output is produced.