this is an issue many have had...
especially on widows the JSON files that holds internal track of lot if the SIA infor for your host & renter are not so stable. I've lost a lot of contracts due to it, but i was able to get the system back running by simly deleting some of those JSON file.... you wil find the folder, if you in the UI do a "show SIA data" - then you will se where the folders are and you might be able to take out the corrupted ones. I cana't tell you which data you will loose, but your funds can always be recovered by sweeping the seed - I've done that many times and works fine.
So After some tests:
I tried to download files with with ratio > 1 -> ok with ratio < 1 -> failed
So my story:
I upload files to backup it
I paid by allowing SCs for the service
I reach a ratio > 2.2 for all files
I close SIA UI for few days
I open it again
Ratio of some files is below 1
My understanding was if a node left network and don't respect the contract I got SCs back + collateral and the network try to find other node to keep ratio high enough.
My mistake was to close the sia-ui for few days, but it isn't very useful to keep a service always up if you just backup files.
I have a node for "selling" space but for each contract I paid for collateral.
Conclusion I paid to backup I paid to share and my backup isn't reliable.