TheBlooner AceTin, Unfortunately, even though it might be a pain, you’ll have to open it back up again! Once opened, unplug the battery and then the other connections you had off before. Inspect the flex cables and connectors closely (a magnifying glass at least, if you don’t have a microscope) and then clean both sides (board and cable) side of the connectors with >99% isopropyl alcohol. Normal rubbing alcohol and the likes will not do, as they mostly contain to many additives to the alcohol.
Once you’ve inspected and cleaned everything very thoroughly, connect everything back (battery last) and check functionality before closing it up again! That last part is learned through experience (I had to open the first few devices I did back up again, because I hadn’t checked and verified functionality).
If it still a problem even though everything looks OK, let us know and we’ll have to dig a bit further.