Even better would be to test if your backup job works with a different storage provider…
TBH, I have no idea whether or not this is a Duplicati or a Jottacloud issue, I am just suspecting that it might be Jottacloud because it has previously been found to be very slow (just search Jottacloud on this forum), which is not surprising, given its unlimited plan.
However, even if something is wrong with Jottacloud, I do agree that duplicati could do a better job in handling that situation: