The server encountered an internal error and was unable to complete your request. Either the server is overloaded or there is an error in the application.

When requesting an attended Sysupgrade, I get an 500 server error when LuCI POSTs to https://sysupgrade.openwrt.org/api/v1/build:

{
  "detail": "The server encountered an internal error and was unable to complete your request. Either the server is overloaded or there is an error in the application.",
  "status": 500,
  "title": "Internal Server Error",
  "type": "about:blank"
}

The build request, which has worked in the past, is as follows:

{
    "url": "https://sysupgrade.openwrt.org",
    "revision": "r23635-03b3284805",
    "advanced_mode": "0",
    "sha256_unsigned": "",
    "branch": "SNAPSHOT",
    "efi": null,
    "profile": "linksys,e8450-ubi",
    "target": "mediatek/mt7622",
    "version": "SNAPSHOT",
    "packages": [
        "auc",
        "base-files",
        "blockd",
        "busybox",
        "ca-bundle",
        "dnsmasq",
        "dropbear",
        "ethtool",
        "firewall4",
        "fstools",
        "fwtool",
        "getrandom",
        "hostapd-common",
        "hostapd-utils",
        "ip-full",
        "iw",
        "iwinfo",
        "jansson",
        "jshn",
        "jsonfilter",
        "kernel",
        "kmod-cfg80211",
        "kmod-crypto-acompress",
        "kmod-crypto-aead",
        "kmod-crypto-ccm",
        "kmod-crypto-cmac",
        "kmod-crypto-crc32",
        "kmod-crypto-crc32c",
        "kmod-crypto-ctr",
        "kmod-crypto-gcm",
        "kmod-crypto-gf128",
        "kmod-crypto-ghash",
        "kmod-crypto-hash",
        "kmod-crypto-hmac",
        "kmod-crypto-manager",
        "kmod-crypto-null",
        "kmod-crypto-rng",
        "kmod-crypto-seqiv",
        "kmod-crypto-sha512",
        "kmod-fs-btrfs",
        "kmod-fs-exfat",
        "kmod-fs-ext4",
        "kmod-fs-f2fs",
        "kmod-fs-ntfs3",
        "kmod-fs-vfat",
        "kmod-fs-xfs",
        "kmod-gpio-button-hotplug",
        "kmod-hwmon-core",
        "kmod-leds-gpio",
        "kmod-lib-crc-ccitt",
        "kmod-lib-crc16",
        "kmod-lib-crc32c",
        "kmod-lib-lzo",
        "kmod-mac80211",
        "kmod-mt76-connac",
        "kmod-mt76-core",
        "kmod-mt7615-common",
        "kmod-mt7615-firmware",
        "kmod-mt7615e",
        "kmod-mt7622-firmware",
        "kmod-mt7915-firmware",
        "kmod-nf-conntrack",
        "kmod-nf-conntrack6",
        "kmod-nf-flow",
        "kmod-nf-log",
        "kmod-nf-log6",
        "kmod-nf-nat",
        "kmod-nf-reject",
        "kmod-nf-reject6",
        "kmod-nfnetlink",
        "kmod-nft-core",
        "kmod-nft-fib",
        "kmod-nft-nat",
        "kmod-nft-offload",
        "kmod-nls-cp437",
        "kmod-nls-iso8859-1",
        "kmod-nls-utf8",
        "kmod-ppp",
        "kmod-pppoe",
        "kmod-pppox",
        "kmod-slhc",
        "kmod-usb-storage-extras",
        "kmod-usb-storage-uas",
        "kmod-usb3",
        "libblobmsg-json",
        "libc",
        "libiwinfo",
        "libiwinfo-data",
        "libjson-c",
        "libjson-script",
        "libmbedtls",
        "libmnl",
        "libnftnl",
        "libnl-tiny",
        "libubox",
        "libubus",
        "libuci",
        "libuclient",
        "libucode",
        "libustream-mbedtls",
        "lldpd",
        "logd",
        "luci",
        "luci-app-attendedsysupgrade",
        "luci-app-wol",
        "luci-i18n-attendedsysupgrade-de",
        "luci-i18n-base-de",
        "luci-i18n-firewall-de",
        "luci-i18n-opkg-de",
        "luci-i18n-wol-de",
        "luci-theme-openwrt-2020",
        "mtd",
        "netifd",
        "nftables-json",
        "odhcp6c",
        "odhcpd-ipv6only",
        "openwrt-keyring",
        "opkg",
        "ppp",
        "ppp-mod-pppoe",
        "procd",
        "procd-seccomp",
        "procd-ujail",
        "tc-full",
        "ubi-utils",
        "uboot-envtools",
        "ubox",
        "ubus",
        "ubusd",
        "uci",
        "uclient-fetch",
        "ucode",
        "ucode-mod-fs",
        "ucode-mod-ubus",
        "ucode-mod-uci",
        "urandom-seed",
        "urngd",
        "usign",
        "wireless-regdb",
        "wpad-mbedtls"
    ],
    "diff_packages": true,
    "filesystem": "squashfs",
    "client": "luci/git-23.132.65998-fa9fb2f"
}

Start by reproducing the LuCI POST to https://sysupgrade.openwrt.org/api/v1/build with the supplied request payload and revision. Trace which server-side handling produces the HTTP 500, then verify that the same attended Sysupgrade request completes successfully without the internal-error response.