Somehow the nexus db got corrupted. This lead to errors like this:

Task ID: 03f9b925-acfa-4d61-a386-a058509d5aed
Task Name: Rebuild maven repository metadata
Stack-trace:
org.sonatype.goodies.common.MultipleFailures$MultipleFailuresException: Failed to run task 'Rebuilding Maven Metadata of cnaf-snapshots'; 1 failure
at org.sonatype.goodies.common.MultipleFailures.maybePropagate(MultipleFailures.java:95)
at org.sonatype.nexus.repository.RepositoryTaskSupport.execute(RepositoryTaskSupport.java:90)
at org.sonatype.nexus.scheduling.TaskSupport.call(TaskSupport.java:100)
at org.sonatype.nexus.quartz.internal.task.QuartzTaskJob.doExecute(QuartzTaskJob.java:143)
at org.sonatype.nexus.quartz.internal.task.QuartzTaskJob.execute(QuartzTaskJob.java:106)
at org.quartz.core.JobRunShell.run(JobRunShell.java:202)
at org.sonatype.nexus.quartz.internal.QuartzThreadPool.lambda$0(QuartzThreadPool.java:145)
at org.sonatype.nexus.thread.internal.MDCAwareRunnable.run(MDCAwareRunnable.java:40)
at org.apache.shiro.subject.support.SubjectRunnable.doRun(SubjectRunnable.java:120)
at org.apache.shiro.subject.support.SubjectRunnable.run(SubjectRunnable.java:108)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
at java.lang.Thread.run(Thread.java:748)
Suppressed: com.orientechnologies.orient.core.exception.OPageIsBrokenException: Following files and pages are detected to be broken ['browse_node.cpm' :16433;], storage is switched to 'read only' mode. Any modification operations are prohibited.
To restore database and make it fully operational you may export and import database to and from JSON.
DB name="component"
at com.orientechnologies.orient.core.storage.impl.local.OAbstractPaginatedStorage.checkLowDiskSpaceRequestsAndReadOnlyConditions(OAbstractPaginatedStorage.java:5144)
at com.orientechnologies.orient.core.storage.impl.local.OAbstractPaginatedStorage.commit(OAbstractPaginatedStorage.java:1729)
at com.orientechnologies.orient.core.tx.OTransactionOptimistic.doCommit(OTransactionOptimistic.java:541)
at com.orientechnologies.orient.core.tx.OTransactionOptimistic.commit(OTransactionOptimistic.java:99)
at com.orientechnologies.orient.core.db.document.ODatabaseDocumentTx.commit(ODatabaseDocumentTx.java:2908)
at com.orientechnologies.orient.core.db.document.ODatabaseDocumentTx.commit(ODatabaseDocumentTx.java:2870)
at org.sonatype.nexus.repository.storage.StorageTxImpl.commit(StorageTxImpl.java:182)
at sun.reflect.GeneratedMethodAccessor100.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at org.sonatype.nexus.common.stateguard.SimpleMethodInvocation.proceed(SimpleMethodInvocation.java:53)
at org.sonatype.nexus.common.stateguard.MethodInvocationAction.run(MethodInvocationAction.java:39)
at org.sonatype.nexus.common.stateguard.StateGuard$TransitionImpl.run(StateGuard.java:193)
at org.sonatype.nexus.common.stateguard.TransitionsInterceptor.invoke(TransitionsInterceptor.java:56)
at org.sonatype.nexus.common.stateguard.StateGuardAspect$1.invoke(StateGuardAspect.java:66)
at com.sun.proxy.$Proxy209.commit(Unknown Source)
at org.sonatype.nexus.transaction.TransactionalWrapper.proceedWithTransaction(TransactionalWrapper.java:68)
at org.sonatype.nexus.transaction.Operations.proceedWithTransaction(Operations.java:232)
at org.sonatype.nexus.transaction.Operations.transactional(Operations.java:219)
at org.sonatype.nexus.transaction.Operations.call(Operations.java:166)
at org.sonatype.nexus.repository.maven.internal.orient.OrientMetadataRebuilder$Worker.rebuildMetadataInner(OrientMetadataRebuilder.java:521)
at org.sonatype.nexus.repository.maven.internal.orient.OrientMetadataRebuilder$Worker.rebuildMetadata(OrientMetadataRebuilder.java:453)
at org.sonatype.nexus.repository.maven.internal.orient.OrientMetadataRebuilder.rebuildInTransaction(OrientMetadataRebuilder.java:156)
at org.sonatype.nexus.repository.maven.internal.orient.OrientMetadataRebuilder.rebuild(OrientMetadataRebuilder.java:132)
at org.sonatype.nexus.repository.maven.internal.orient.MavenHostedFacetImpl.rebuildMetadata(MavenHostedFacetImpl.java:117)
at org.sonatype.nexus.repository.maven.tasks.RebuildMaven2MetadataTask.execute(RebuildMaven2MetadataTask.java:60)
at org.sonatype.nexus.repository.RepositoryTaskSupport.execute(RepositoryTaskSupport.java:79)
... 13 more

It was fixed following this recipe:

https://gist.github.com/marcelmaatkamp/123e8793e07a72a382d8d0e8d66bbd8f

This task is here for future reference