indexes in subquery causing MariaDB crash ???

where to begin.....

Over the last 24 hours i've had 3 (out of 5 total) mariadb production servers crash with similar errors (below).

Each server has about 30 customer databases (identical structure) that customers connect to from an indentical win32 desktop using ODBC.

INNODB tables.

Not aware of any changes to the servers or the databases or the win32.exe.

On each server there has been, since yesterday, one database where if i run a couple of specific but seemingly innocuous queries the server crashes and restarts.

The error mentions an index.... so in each of the queries i (eventually) removed the indexes used by a subquery. and hey presto no crash. slow but no crash.

I must emphasise that i only removed the indexes from the one database on each server that was mentioned in the crash report. The other databases with the same structure on the same servers continue to work just fine with the same queries and the same indexes . And the database that are causing a crash have been same structure running same queries for months and months.

the databases causing the error are no larger than any of the others (a few thousand records in the deal and invoices tables).

i removed the indexes on invoices,dealid invoices.who invoices.term invoices.invoice deal.dealid

.dealid and .invoice and .term are int 11; .who is varchar 10

the other query that causes a crash is an update and uses invoice.invoice in a subquery

again, to emphasise, this query has been run a thousand times a day on 200 or more databases for more than a year with no problem... until yesterday on 3 databases

HELP !!!!

NIgel