/home/travis/server/10.4/bin/mariabackup based on MariaDB server 10.4.2-MariaDB Linux (x86_64) 
mariabackup: cd to /home/travis/logs/vardir3/backup/
Removing ./test/t4.ibd
Renaming ./test/t4.new to ./test/t4.ibd
Removing ./test/seq10.ibd
Renaming ./test/seq10.new to ./test/seq10.ibd
Renaming ./test/#sql-6e62_10.new to ./test/#sql-6e62_10.ibd
ERROR : File ./test/seq5.ren was not found, but expected during rename processing
2019-01-09 16:19:22 0x7f137aeeb740  InnoDB: Assertion failure in file /data/src/10.4/extra/mariabackup/xtrabackup.cc line 5390
InnoDB: Failing assertion: 0
InnoDB: We intentionally generate a memory trap.
InnoDB: Submit a detailed bug report to https://jira.mariadb.org/
InnoDB: If you get repeated assertion failures or crashes, even
InnoDB: immediately after the mysqld startup, there may be
InnoDB: corruption in the InnoDB tablespace. Please refer to
InnoDB: https://mariadb.com/kb/en/library/innodb-recovery-modes/
InnoDB: about forcing recovery.
190109 16:19:22 [ERROR] mysqld got signal 6 ;
This could be because you hit a bug. It is also possible that this binary
or one of the libraries it was linked against is corrupt, improperly built,
or misconfigured. This error can also be caused by malfunctioning hardware.

To report this bug, see https://mariadb.com/kb/en/reporting-bugs

We will try our best to scrape up some info that will hopefully help
diagnose the problem, but since we have already crashed, 
something is definitely wrong and this may fail.

Server version: 10.4.2-MariaDB-debug
key_buffer_size=0
read_buffer_size=131072
max_used_connections=0
max_threads=1
thread_count=0
It is possible that mysqld could use up to 
key_buffer_size + (read_buffer_size + sort_buffer_size)*max_threads = 5736 K  bytes of memory
Hope that's ok; if not, decrease some variables in the equation.

Thread pointer: 0x0
Attempting backtrace. You can use the following information to find out
where mysqld died. If you see no messages after this, something went
terribly wrong...
stack_bottom = 0x0 thread_stack 0x49000
mysys/stacktrace.c:269(my_print_stacktrace)[0x5556ffc79567]
sql/signal_handler.cc:168(handle_fatal_signal)[0x5556ff5545e8]
/lib/x86_64-linux-gnu/libpthread.so.0(+0x110c0)[0x7f137aade0c0]
linux/raise.c:51(__GI_raise)[0x7f137901ffcf]
stdlib/abort.c:91(__GI_abort)[0x7f13790213fa]
ut/ut0mem.cc:42(ut_strlcpy(char*, char const*, unsigned long))[0x5556ffafcedf]
mariabackup/xtrabackup.cc:5392(rename_table_in_prepare(std::string const&, std::string const&, std::string const&, char const*))[0x5556ff04f9c1]
mariabackup/xtrabackup.cc:5378(rename_table_in_prepare(std::string const&, std::string const&, std::string const&, char const*))[0x5556ff04f6e7]
mariabackup/xtrabackup.cc:5414(prepare_handle_ren_files(char const*, char const*, char const*, void*))[0x5556ff04ff13]
mariabackup/xtrabackup.cc:5269(xb_process_datadir(char const*, char const*, unsigned long (*)(char const*, char const*, char const*, void*)))[0x5556ff04f269]
mariabackup/xtrabackup.cc:5464(xtrabackup_prepare_func(char**))[0x5556ff0504ea]
mariabackup/xtrabackup.cc:6418(main_low(char**))[0x5556ff052bda]
mariabackup/xtrabackup.cc:6211(main)[0x5556ff0523ee]
csu/libc-start.c:325(__libc_start_main)[0x7f137900d2b1]
/home/travis/server/10.4/bin/mariabackup(_start+0x2a)[0x5556ff0423fa]
The manual page at http://dev.mysql.com/doc/mysql/en/crashing.html contains
information that should help you find out what is causing the crash.
Aborted