What’s the actual error you're getting when making the failed calls?

The stderr reports for last 3 chunks of this file:

+ CHUNK_PARAMS='upload_id=0B3fybsi14V1MmpZ18As_Q&offset=209715200'
++ utime
+++ date +%s
++ echo 1434394232
+ curl -s --show-error --globoff -i -o /tmp/du_resp_debug --upload-file /tmp/du_chunk_1035 'https://api-content.dropbox.com/1/chunked_upload?upload_id=0B3fybsi14V1MmpZ18As_Q&offset=209715200&oauth_consumer_key=xeg3vxakd86svsh&oauth_token=jm624faonumhhxl9&oauth_signature_method=PLAINTEXT&oauth_signature=<redacted>%<redacted>&oauth_timestamp=1434394232&oauth_nonce=14453'
+ check_http_response
+ CODE=0
+ case $CODE in
+ grep -q 'HTTP/1.1 400' /tmp/du_resp_debug
+ grep -q '^HTTP/1.1 200 OK' /tmp/du_resp_debug
+ print '*'
+ [[ 0 == 0 ]]
+ echo -ne '*'
+ let UPLOAD_ERROR=0+1
+ [[ 1 -gt 2 ]]
+ [[ 209715200 != 524288000 ]]
+ let OFFSET_MB=209715200/1024/1024
+ dd if=/var/lib/condor/execute/dir_2220/oneA.e15.oswrk136_500GB.data of=/tmp/du_chunk_1035 bs=1048576 skip=200 count=100
+ [[ 209715200 != 0 ]]
+ CHUNK_PARAMS='upload_id=0B3fybsi14V1MmpZ18As_Q&offset=209715200'
++ utime
+++ date +%s
++ echo 1434394296
+ curl -s --show-error --globoff -i -o /tmp/du_resp_debug --upload-file /tmp/du_chunk_1035 'https://api-content.dropbox.com/1/chunked_upload?upload_id=0B3fybsi14V1MmpZ18As_Q&offset=209715200&oauth_consumer_key=xeg3vxakd86svsh&oauth_token=jm624faonumhhxl9&oauth_signature_method=PLAINTEXT&oauth_signature=<redacted>%<redacted>&oauth_timestamp=1434394296&oauth_nonce=5737'
+ check_http_response
+ CODE=0
+ case $CODE in
+ grep -q 'HTTP/1.1 400' /tmp/du_resp_debug
++ sed -n -e 's/{"error": "\([^"]*\)"}/\1/p' /tmp/du_resp_debug
+ ERROR_MSG=
+ case $ERROR_MSG in
+ grep -q '^HTTP/1.1 200 OK' /tmp/du_resp_debug
+ print '*'
+ [[ 0 == 0 ]]
+ echo -ne '*'
+ let UPLOAD_ERROR=1+1
+ [[ 2 -gt 2 ]]
+ [[ 209715200 != 524288000 ]]
+ let OFFSET_MB=209715200/1024/1024
+ dd if=/var/lib/condor/execute/dir_2220/oneA.e15.oswrk136_500GB.data of=/tmp/du_chunk_1035 bs=1048576 skip=200 count=100
+ [[ 209715200 != 0 ]]
+ CHUNK_PARAMS='upload_id=0B3fybsi14V1MmpZ18As_Q&offset=209715200'
++ utime
+++ date +%s
++ echo 1434394336
+ curl -s --show-error --globoff -i -o /tmp/du_resp_debug --upload-file /tmp/du_chunk_1035 'https://api-content.dropbox.com/1/chunked_upload?upload_id=0B3fybsi14V1MmpZ18As_Q&offset=209715200&oauth_consumer_key=xeg3vxakd86svsh&oauth_token=jm624faonumhhxl9&oauth_signature_method=PLAINTEXT&oauth_signature=<redacted>%<redacted>&oauth_timestamp=1434394336&oauth_nonce=21291'
+ check_http_response
+ CODE=0
+ case $CODE in
+ grep -q 'HTTP/1.1 400' /tmp/du_resp_debug
++ sed -n -e 's/{"error": "\([^"]*\)"}/\1/p' /tmp/du_resp_debug
+ ERROR_MSG=
+ case $ERROR_MSG in
+ grep -q '^HTTP/1.1 200 OK' /tmp/du_resp_debug
+ print '*'
+ [[ 0 == 0 ]]
+ echo -ne '*'
+ let UPLOAD_ERROR=2+1
+ [[ 3 -gt 2 ]]
+ print ' FAILED\n'
+ [[ 0 == 0 ]]
+ echo -ne ' FAILED\n'
+ print 'An error occurred requesting /chunked_upload\n'
+ [[ 0 == 0 ]]
+ echo -ne 'An error occurred requesting /chunked_upload\n'
+ ERROR_STATUS=1
+ return
+ (( i++  ))
+ (( i<6 ))
+ remove_temp_files
+ [[ 1 == 0 ]]
+ exit 1

====================================================================================

====================================================================================

What chunk size are you using?

CHUNK_SIZE=100

It is in MB. The code sending data looks like this:

#Chunked file upload
#$1 = Local source file
#$2 = Remote destination file
function db_chunked_upload_file
{
    local FILE_SRC=$(normalize_path "$1")
    local FILE_DST=$(normalize_path "$2")

    print " > Uploading \"$FILE_SRC\" to \"$FILE_DST\"

    local FILE_SIZE=$(file_size "$FILE_SRC")
    local OFFSET=0
    local UPLOAD_ID="
    local UPLOAD_ERROR=0

    local CHUNK_PARAMS="

    #Uploading chunks...
    while ([[ $OFFSET != $FILE_SIZE ]]); do

        let OFFSET_MB=$OFFSET/1024/1024

        #Create the chunk
        dd if="$FILE_SRC" of="$CHUNK_FILE" bs=1048576 skip=$OFFSET_MB count=$CHUNK_SIZE 2> /dev/null

        #Only for the first request these parameters are not included
        if [[ $OFFSET != 0 ]]; then
            CHUNK_PARAMS="upload_id=$UPLOAD_ID&offset=$OFFSET"
        fi

        #Uploading the chunk...
        $CURL_BIN $CURL_ACCEPT_CERTIFICATES -s --show-error --globoff -i -o "$RESPONSE_FILE" --upload-file "$CHUNK_FILE" "$API_CHUNKED_UPLOAD_URL?$CHUNK_PARAMS&oauth_consumer_key=$APPKEY&oauth_token=$OAUTH_ACCESS_TOKEN&oauth_signature_method=PLAINTEXT&oauth_signature=$APPSECRET%26$OAUTH_ACCESS_TOKEN_SECRET&oauth_timestamp=$(utime)&oauth_nonce=$RANDOM" 2> /dev/null
        check_http_response

        #Check
        if grep -q "^HTTP/1.1 200 OK" "$RESPONSE_FILE"; then
            print "."
            UPLOAD_ERROR=0
            UPLOAD_ID=$(sed -n 's/.*"upload_id": *"*\([^"]*\)"*.*/\1/p' "$RESPONSE_FILE")
            OFFSET=$(sed -n 's/.*"offset": *\([^}]*\).*/\1/p' "$RESPONSE_FILE")
        else
            print "*"
            let UPLOAD_ERROR=$UPLOAD_ERROR+1

            #On error, the upload is retried for max 3 times
            if [[ $UPLOAD_ERROR -gt 2 ]]; then
                print " FAILED\n"
                print "An error occurred requesting /chunked_upload\n"
                ERROR_STATUS=1
                return
            fi
        fi

    done

    UPLOAD_ERROR=0

    #Commit the upload
    while (true); do

        $CURL_BIN $CURL_ACCEPT_CERTIFICATES -s --show-error --globoff -i -o "$RESPONSE_FILE" --data "upload_id=$UPLOAD_ID&oauth_consumer_key=$APPKEY&oauth_token=$OAUTH_ACCESS_TOKEN&oauth_signature_method=PLAINTEXT&oauth_signature=$APPSECRET%26$OAUTH_ACCESS_TOKEN_SECRET&oauth_timestamp=$(utime)&oauth_nonce=$RANDOM" "$API_CHUNKED_UPLOAD_COMMIT_URL/$ACCESS_LEVEL/$(urlencode "$FILE_DST")" 2> /dev/null
        check_http_response

        #Check
        if grep -q "^HTTP/1.1 200 OK" "$RESPONSE_FILE"; then
            print "."
            UPLOAD_ERROR=0
            break
        else
            print "*"
            let UPLOAD_ERROR=$UPLOAD_ERROR+1

            #On error, the commit is retried for max 3 times
            if [[ $UPLOAD_ERROR -gt 2 ]]; then
                print " FAILED\n"
                print "An error occurred requesting /commit_chunked_upload\n"
                ERROR_STATUS=1
                return
            fi
        fi

    done

    print " DONE\n"
}