Thanks Andre. Apologies, my mistake: It's Sony NW ZX5057walkman, which has full Android functionality. I am not aware of an update, although the wifi was open. The issues started happening, strangely, very suddenly. Not sure about the skin, but I did not change anything between the moment it worked and the moment it did not work. Whatever the skin was, it was the same when the app was working fine (I worked wonderfully for a month with no bugs at all). The software version for the Walkman is 4.04.00, but it does not say when the latest update was. The one odd thing I noticed when Poweramp stopped working well was that the Sony Music Up was updating the database. But I can´t see why that is conneceted to Poweramp: the database in Poweramp is correct and it has even the most recent additions to the system library.
The worst thing I guess is that the "Albums" section in the library takes much longer to open than it used to (I have disabled the scan everytime the app opens, as I have not been adding any music).