What's going on at my 5th fret?

Check both neck relief and action.

If strings travel too far when fretting, notes can go sharp.

 

Maybe try different strings?

Does this happen with all the strings?

 

"Relief" means concave. It gives the middle of the string a little room to vibrate. If the neck is too straight, you need to set the action high to keep the string from vibrating against the frets.

If the action is high, you stretch the string when you fret at the 12th fret, pulling it sharp. If you then adjust the intonation to compensate for that, lower notes may be flat because fretting them doesn't stretch the string as much.

If the action is low and there's not enough relief, a string may vibrate slightly against the frets, causing it to go sharp.

After 15 months of not being played, your relief may have changed. Or the strings could have corroded, which may also cause them to play out of tune. A new set of strings and good setup with them will probably solve your problems, not to mention you'll love the way it sounds.

 

*How are you setting intonation? 12th harmonic vs 12th fretted? From the 5th and below: flat or sharp?

Riis

 

Does the 12th harmonic match the 12th fretted? If yes, what happens at the 6th fret and up?

Riis