The recycle bin of the media section is empty...

Have you done any content/data migrations recently?

In my experience, media items don't just disappear they are lost and it is common to lose media between versions when doing a migration.

Do you have any backups?

Nope, no one data migrations recently.

The only thing that I made was to delete the App_Data/cache folder some days ago.

Can be a problem?

But the strange thing is that the missing images are from the media folder, not from the cache...

Sorry to say that if images go missing from the disk, its not usually an Umbraco issue, do you have any custom Media service code or Plugins that manage any part of the media folder?

Deleting the cache wont cause this issue.

do you have any custom Media service code or Plugins that manage any part of the media folder?

No, I don't have any plugins installed on Umbraco, or service code...

About this question:

Have you done any content/data migrations recently?

The only thing that I've done recently was to duplicate an entire node (with many children) to create another language version of the website.

During this copy of content I've received many errors like this:

System.Data.SqlServerCe.SqlCeException: There is a file sharing violation. A different process might be using the file.

This can be the problem?

Could well be, I would start there and see if you can dig into the logs.

Thanks

Can I upload the log file?

Here are some errors from the log file:

2018-02-21 18:42:41,088 [P2892/D4/T32] ERROR Umbraco.Web.Strategies.ServerRegistrationEventHandler - Failed to update server record in database.
System.Data.SqlServerCe.SqlCeException (0x80004005): There is a file sharing violation. A different process might be using the file. [ ..\App_Data\dbUmbMod.sdf ]
   in System.Data.SqlServerCe.SqlCeConnection.Open(Boolean silent)
   in System.Data.SqlServerCe.SqlCeConnection.Open()
   in Umbraco.Core.Persistence.FaultHandling.RetryPolicy.<>c__DisplayClass1.<ExecuteAction>b__0()
   in Umbraco.Core.Persistence.FaultHandling.RetryPolicy.ExecuteAction[TResult](Func`1 func)
   in Umbraco.Core.Persistence.Database.OpenSharedConnection()
   in Umbraco.Core.Persistence.Database.BeginTransaction(IsolationLevel isolationLevel)
   in Umbraco.Core.Scoping.Scope.get_Database()
   in Umbraco.Core.Persistence.LockingRepository`1.WithWriteLocked(Action`1 action, Boolean autoCommit)
   in Umbraco.Core.Services.ServerRegistrationService.TouchServer(String serverAddress, String serverIdentity, TimeSpan staleTimeout)
   in Umbraco.Web.Strategies.ServerRegistrationEventHandler.TouchServerTask.PerformRun()