The player may be interpreting the subtitle file with the wrong character encoding, or the source text may already be damaged. Language selection and font support can also matter. Compare another track and identify whether the problem affects all text or only a particular programme.
Separate missing glyphs from wrong decoding
Empty boxes can indicate that the selected font lacks characters. Sequences of unrelated symbols where accented letters should be can indicate an encoding mismatch. These are different issues. A screenshot showing a short example can help support, provided it does not expose private account information or unrelated personal content.
Use supported text options
If the player documents a subtitle-encoding control, record its original value and try the appropriate supported encoding for the file. Do not cycle randomly through settings and save the first partially readable result. For integrated service captions, a damaged source track may need correction by the provider rather than a local workaround.
Check the whole language
Test several lines containing punctuation, names and non-Latin characters if relevant. A setting that repairs one accented word can still break another part of the track. Keep a readable fallback and report the title, language, player version and example time. Reinstalling the entire app is rarely a useful first step when every other track displays normally.