Update caches not reachable

Hi, we have two update caches, on separate sites, which have been working until last week. Both update caches stopped serving requests at around the same time. Uninstalling and re-installing has not solved the problem.

The relevant lines, with customer info removed, from the client update log are:

2018-03-14T15:39:51.702Z [ 4568] [v5.11.141] INFO  Analyzing whether to update from Sophos CDN or update cache
2018-03-14T15:39:51.973Z [ 4568] [v5.11.141] INFO  Checking access to update cache: updatecachefqdn:8191 using customer file e/ae/eae0cfc7681faacacaf3d47030b47e2614ef7690f3b9f14bfa4163ef6238f072.dat
2018-03-14T15:39:54.240Z [ 4568] [v5.11.141] WARN  [W23245] 500 Internal Server Error: http://updatecachefqdn:8191/sophos/customer/e/ae/eae0cfc7681faacacaf3d47030b47e2614ef7690f3b9f14bfa4163ef6238f072.dat
2018-03-14T15:39:54.241Z [ 4568] [v5.11.141] INFO  [I23317] No proxy was used.
2018-03-14T15:39:54.241Z [ 4568] [v5.11.141] WARN  [W41450] 500 Internal Server Error: http://updatecachefqdn:8191/sophos/customer/e/ae/eae0cfc7681faacacaf3d47030b47e2614ef7690f3b9f14bfa4163ef6238f072.dat
2018-03-14T15:39:54.241Z [ 4568] [v5.11.141] INFO  Could not reach cache: updatecachefqdn:8191
2018-03-14T15:39:54.244Z [ 4568] [v5.11.141] INFO  Checking access to update cache:updatecachefqdn:8191 using customer file 2/cd/2cdbf7501b2ff37ba1c392a1fa5d16ab.dat
2018-03-14T15:39:55.842Z [ 4568] [v5.11.141] WARN  [W23245] 500 Internal Server Error: http://updatecachefqdn:8191/sophos/customer/2/cd/2cdbf7501b2ff37ba1c392a1fa5d16ab.dat
2018-03-14T15:39:55.842Z [ 4568] [v5.11.141] INFO  [I23317] No proxy was used.
2018-03-14T15:39:55.842Z [ 4568] [v5.11.141] WARN  [W41450] 500 Internal Server Error: http://updatecachefqdn:8191/sophos/customer/2/cd/2cdbf7501b2ff37ba1c392a1fa5d16ab.dat
2018-03-14T15:39:55.842Z [ 4568] [v5.11.141] INFO  Could not reach cache:updatecachefqdn:8191

Any ideas?

Thanks.